[Gemfile] Add flag to skip unreleased versions

This can be set to true on release branches, so that only versions of the gems which were released and pushed to RubyGems are considered.

Gemfile

+SKIP_UNRELEASED_VERSIONS = false
+
 # Declares a dependency to the git repo of CocoaPods gem. This declaration is
 # compatible with the local git repos feature of Bundler.
 #
 def cp_gem(name, repo_name, branch = 'master', path: false)
+  return gem name if SKIP_UNRELEASED_VERSIONS
   opts = if path
            { :path => "../#{repo_name}" }
          else