fixed #12 - Set task message size to be a configurable option

Adds a new option maxPayloadSize, which takes a number in bytes that is the maximum size of a message payload to be sent across the notification bus.
Added redis to travis configuration
Fixed travis config indentation
Merge pull request #13 from Kinvey/12-configurable-task-message-size

fixed #12 - Set task message size to be a configurable option