google-cloud-ruby: LoadError (google/gax) when using PubSub

Hi, I am getting an error when trying to use google/cloud/pubsub.

$ bundle exec bin/ght-mongo-stream config.yaml 
/var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub/v1/publisher_api.rb:28:in `require': cannot load such file -- google/gax (LoadError)
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub/v1/publisher_api.rb:28:in `<top (required)>'
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub/v1.rb:16:in `require'
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub/v1.rb:16:in `<top (required)>'
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub/service.rb:19:in `require'
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub/service.rb:19:in `<top (required)>'
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub/project.rb:18:in `require'
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub/project.rb:18:in `<top (required)>'
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub.rb:17:in `require'
	from /var/lib/gems/2.2.0/gems/google-cloud-pubsub-0.21.0/lib/google/cloud/pubsub.rb:17:in `<top (required)>'
	from /home/gousiosg/ghtorrent-streaming/lib/streaming/gpubsub.rb:2:in `require'
	from /home/gousiosg/ghtorrent-streaming/lib/streaming/gpubsub.rb:2:in `<top (required)>'
	from /home/gousiosg/ghtorrent-streaming/lib/streaming.rb:7:in `require'
	from /home/gousiosg/ghtorrent-streaming/lib/streaming.rb:7:in `<top (required)>'
	from bin/ght-mongo-stream:9:in `require'
	from bin/ght-mongo-stream:9:in `<main>'

My Gemfile.lock contains the google-gax (0.6.0)

PATH
  remote: .
  specs:
    streaming-streaming (0.1.0)

GEM
  remote: https://rubygems.org/
  specs:
    addressable (2.4.0)
    amq-protocol (2.0.1)
    azure (0.7.1)
      addressable (~> 2.3)
      faraday (~> 0.9)
      faraday_middleware (~> 0.10)
      json (~> 1.8)
      mime-types (>= 1, < 3.0)
      nokogiri (~> 1.6)
      systemu (~> 2.6)
      thor (~> 0.19)
      uuid (~> 2.0)
    bson (1.12.5)
    bson_ext (1.12.5)
      bson (~> 1.12.5)
    bunny (2.2.2)
      amq-protocol (>= 2.0.1)
    faraday (0.9.2)
      multipart-post (>= 1.2, < 3)
    faraday_middleware (0.10.0)
      faraday (>= 0.7.4, < 0.10)
    google-cloud-core (0.21.1)
      googleauth (~> 0.5.1)
    google-cloud-pubsub (0.21.0)
      google-cloud-core (~> 0.21.0)
      google-gax (~> 0.6.0)
      google-protobuf (~> 3.0)
      googleapis-common-protos (~> 1.3)
      grpc (~> 1.0)
      grpc-google-iam-v1 (~> 0.6.8)
    google-gax (0.6.0)
      googleapis-common-protos (~> 1.3.1)
      googleauth (~> 0.5.1)
      grpc (~> 1.0)
      rly (~> 0.2.3)
    google-protobuf (3.0.2)
    googleapis-common-protos (1.3.4)
      google-protobuf (~> 3.0)
      grpc (~> 1.0)
    googleauth (0.5.1)
      faraday (~> 0.9)
      jwt (~> 1.4)
      logging (~> 2.0)
      memoist (~> 0.12)
      multi_json (~> 1.11)
      os (~> 0.9)
      signet (~> 0.7)
    grpc (1.0.1)
      google-protobuf (~> 3.0.2)
      googleauth (~> 0.5.1)
    grpc-google-iam-v1 (0.6.8)
      googleapis-common-protos (~> 1.3.1)
      googleauth (~> 0.5.1)
      grpc (~> 1.0)
    json (1.8.3)
    jwt (1.5.6)
    little-plugger (1.1.4)
    log4r (1.1.10)
    logging (2.1.0)
      little-plugger (~> 1.1)
      multi_json (~> 1.10)
    macaddr (1.7.1)
      systemu (~> 2.6.2)
    memoist (0.15.0)
    mime-types (2.99.1)
    mini_portile2 (2.0.0)
    mongo (1.12.5)
      bson (= 1.12.5)
    mongoriver (0.4.3)
      bson_ext
      log4r
      mongo (>= 1.7)
    multi_json (1.12.1)
    multipart-post (2.0.0)
    nokogiri (1.6.7.2)
      mini_portile2 (~> 2.0.0.rc2)
    os (0.9.6)
    rake (10.5.0)
    rly (0.2.3)
    signet (0.7.3)
      addressable (~> 2.3)
      faraday (~> 0.9)
      jwt (~> 1.5)
      multi_json (~> 1.10)
    systemu (2.6.5)
    thor (0.19.1)
    uuid (2.3.8)
      macaddr (~> 1.0)

PLATFORMS
  ruby

DEPENDENCIES
  azure
  bson_ext (~> 1.12.5)
  bundler (~> 1.10)
  bunny
  google-cloud-pubsub
  mongo (~> 1.12.5)
  mongoriver
  rake (~> 10.0)
  streaming-streaming!

BUNDLED WITH
   1.11.2

The code that triggers the error is here: https://github.com/ghtorrent/ghtorrent-streaming/blob/master/lib/streaming/gpubsub.rb