Fix required ruby version to >= 1.9.3

The prior version specification `'~>'` was limiting ruby versions to 1.9.X and so ruby 2.0 and 2.1 were not compatible.
See http://guides.rubygems.org/specification-reference/#required_ruby_version=
Merge pull request #24 from tgautier/patch-1

Fix required ruby version to >= 1.9.3