gh: Pull request compatibility

* Use octokit/go-octokit instead of my fork jingweno/go-octokit
* Unify error messages according to the hub wordings
* Remove deprecated hub cukes
* Get most cukes passing in pull_request.feature

Two more cukes to worry about which I'll do that in another PR:

```
Failing Scenarios:
cucumber features/pull_request.feature:116 # Scenario: Failed pull request preserves previous message
cucumber features/pull_request.feature:424 # Scenario: Create pull request to "upstream" remote
```