Add Connection#cancel

- Changed Api::Response to handle optional and fewer fields
- Tested both failure response and success for #cancel
- Add cancellation path to Circle CI environment
- Add cancel path environment variable to Travis
- Remove redundant connection_v03 spec
Move Connection#post and #cancel fully into shared examples
Make method names internally consistent

Rename valid_failure_response to post_failed_response_body to be consistent with cancel_failed_response_body.