Fixing Serialization

In Guzzle 3.7 the Response started implementing the Serializable interface.
This affected the serialization of our responses, since data and metadata were now dropped.
Updated this by moving response content parsing into the response objects, allowing
unserialize method to properly rebuild the response.

Updating Dev dependencies
Updating Travis config.
Merge pull request #7 from rdohms/6-serialization

Fixing Serialization