Fix multiple Curl calls while posting to API

Hello again,

I've found myself using twitch-rb again, and I have encountered an error.

While trying to get an OAuth access token, every POST request returned Error 400.
I tracked this down to the post method, it tries to use the auth code twice, the first valid response is thrown away, and returns the second now-invalid call.

I've fixed up the method, as well as updated the gemfile (no longer required to source rubygems) and updated the gemspec for 0.0.3. I would be very grateful if you could push this release to RubyGems.