Support later versions of RubyGems

Newer versions of RubyGems no longer utilise latest_version? to check if they should install the latest. Now they standardise on creating a dependency with a version requirement '>= 0'.

This update refactors the monkey patch to wrap the Dependency constructor through replacement subclassing and uses a Requirement#none? check to determine an unversioned request.

Explicit false needed to abandon gem install in plugins

To be strictly compatible with pre_install blocks nil no longer terminates the gem install

Initial release

Framework for detecting unversioned gem installations.