Add makeRubyLock builtin

[dsalaza4]

makeRubyGemsEnvironment requires to specify a rubyGems parameter with the entire pinned dependency tree. This is a huge headache as one has to go through each dependency in RubyGems and get its sha256. Aditionally, one still has to get a Gemfile.lock just to know the dependency tree for a set of direct dependencies.

Let's do what we do with Python:

1. Add a makeRubyLock (like makePythonLock) that automatically generates a lock file.
2. Make makeRubyGemsEnvironment receive such lock to create the environment.

[dsalaza4]

rubygems/rubygems#3379