Gemspec updated

There are multiple PRs to update the Gemspec, but they missed that it's generated with Thor. Here an updated version:

* Declare License: #72 , #89, #90 
* Remove Gemfile.lock from the repository: #81 

Also, it declares all development dependencies (including the missing thor & git) in the gemspec, not the Gemfile & removed the like to the late rubyforge project.