add Gemfile

Allows to git clone + bundle install + bundle exec.

Separate version file is needed to fix this error:

    $ bundle install
    There was a LoadError while loading moob.gemspec: 
    cannot load such file -- patron from                                                                                                                                                                                                                                                      
      /home/lex/repos/moob/moob.gemspec:6:in `<main>'