mailcatcher: mailcatcher failed to start

Following the installation instructions I get this error on Ubuntu 12.04 and Ubuntu 14.04:

$ mailcatcher 
/opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- i18n/core_ext/string/interpolate (LoadError)
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-3.2.19/lib/active_support/core_ext/string/interpolation.rb:2:in `<top (required)>'
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-3.2.19/lib/active_support/core_ext/string.rb:9:in `<top (required)>'
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-3.2.19/lib/active_support/core_ext.rb:2:in `block in <top (required)>'
    from /opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-3.2.19/lib/active_support/core_ext.rb:1:in `each'
    from /opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-3.2.19/lib/active_support/core_ext.rb:1:in `<top (required)>'
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mailcatcher-0.5.12/lib/mail_catcher.rb:1:in `<top (required)>'
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /opt/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/mailcatcher-0.5.12/bin/mailcatcher:3:in `<top (required)>'
    from /opt/rbenv/versions/2.1.2/bin/mailcatcher:23:in `load'
    from /opt/rbenv/versions/2.1.2/bin/mailcatcher:23:in `<main>'

List of installed gems:

$ gem list --local

*** LOCAL GEMS ***

activesupport (3.2.19)
bigdecimal (1.2.4)
bundler (1.6.1)
daemons (1.1.9)
eventmachine (1.0.3)
haml (4.1.0.beta.1)
i18n (0.7.0.beta1)
io-console (0.4.2)
json (1.8.1)
mail (2.6.1)
mailcatcher (0.5.12)
mime-types (2.3)
minitest (4.7.5)
multi_json (1.10.1)
psych (2.0.5)
rack (1.6.0.beta)
rack-protection (1.5.3)
rake (10.1.0)
rdoc (4.1.0)
sinatra (1.4.5)
skinny (0.2.3)
sqlite3 (1.3.9)
test-unit (2.1.2.0)
thin (1.5.1)
tilt (1.4.1)

Any ideas?