Remove development dependency for production

The latest gem requires 'capybara_minitest_spec' in capybara-extensions.rb file, this cause Capybara breaks. 
```
cannot load such file -- capybara_minitest_spec (LoadError)
/Users/benjis/.rvm/gems/ruby-2.0.0-p247@cucumber/gems/capybara-extensions-0.3.2/lib/capybara-extensions.rb:2:in `require'
```

I assume it is only needed for development.