Fix timeout and lock rest-client to 1.6.x

This PR makes the following changes to right_api_client:
- Locks the 'rest-client' gem to 1.6.x. Previously it was only locked to 1.x, which allowed end users to pull in rest-client 1.7.x. Unfortunately, right_api_client still needs to support Ruby 1.8.7, which isn't compatible with rest-client 1.7.x.
- Undoes setting of `DEFAULT_TIMEOUT` to `nil`, since this setting doesn't work in rest-client 1.6.x. We revert back to providing -1.