specify ruby version and change to use https rubygems source

Change it because now default on heroku is ruby 2.0.0 and he generate error:
```ruby
State changed from crashed to starting
2014-04-27T20:47:47.721042+00:00 app[web.1]: ruby: symbol lookup error: /app/vendor/bundle/ruby/2.0.0/gems/eventmachine-1.0.0/lib/rubyeventmachine.so: undefined symbol: rb_enable_interrupt
```

after correction work perfectly :smiley: