Follow RubyGems matching specs

* Avoid matching pre-release version if the constraint does
  not contain a pre-release;
* This doesn't apply to < and <= operators.
Streamline the Gemfile
Update implemented SemVer version in README.

Also uUpdate Code Climate badge.
Merge pull request #22 from jilion/follow-rubygems-specs

Pre-release versions should satisfy non-pre-release versions