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.

This caused the following error:
```
Installing cucumber-api-steps (0.12) 
Gem::InstallError: cucumber-api-steps requires Ruby version ~> 1.9.3.
An error occurred while installing cucumber-api-steps (0.12), and Bundler cannot continue.
Make sure that `gem install cucumber-api-steps -v '0.12'` succeeds before bundling.
```

See http://guides.rubygems.org/specification-reference/#required_ruby_version=