Do not specify version constraints explicitly

* http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
No need to require "rspec"
Pry is already in the gemspec
Explicitly require v1.9.3 or above
Added extra Ruby versions to test plan
Merge pull request #2 from sgerrand/require-ruby-version-1.9.3

Require Ruby version 1.9.3