dinghy: Errors with fsevents_to_vm installation on setup

The FseventsToVm#install_if_necessary! method effectively runs this command:

sudo -H /System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/gem install --no-rdoc --no-ri -n /usr/local/bin fsevents_to_vm -v ~> 1.1.3

A couple of problems here,

1. The VERSION constant has a “~” ~which is expanded by the shell to my home directory, forming an invalid command. Changing this to a fixed version fixed this for me.~

2. The system version of the gem install command on Mac OS Sierra is from ruby 2.0. This is too old and causes problems when there are already any installed gem executables in /usr/local/bin, because ruby 2.0 doesn’t recognize the newer executable format installed by later rubies when Gem::Installer#check_executable_overwrite inspects them:

/Library/Ruby/Gems/2.0.0/gems/thor-0.19.4/thor.gemspec
Exception `NoMethodError' at /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/package/tar_reader.rb:71 - undefined method `seek' for #<Zlib::GzipReader:0x007f92dd203bc8>
Exception `TypeError' at /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/installer.rb:171 - no implicit conversion of nil into String
ERROR:  While executing gem ... (TypeError)
    no implicit conversion of nil into String
	/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/installer.rb:171:in `check_executable_overwrite'

(thor is a dependency that is already installed, and causing an error here.)

We could force-overwrite executables with gem install -f, but maybe this is not the best solution. Does this need to be installed via the system version of rubygems or can we use a preferred version?

(e.g. brew installed ruby or ruby-install/chruby/rvm)