Make CI work on pull requests from developer forks, provide messages about needed ENV vars

- Use anonymous Octokit calls up to 60/hr if it detects a pull request from a developer fork which do not permit travis secure environment variables.
- Also if there is not a secure key in .travis.yml for them
- Refers to doco about how to do the travis-encrypted GitHub API key and environment variable
- mvp_spec.rb raises error for invalid key and provides warnings for nil key, which still works

- This will be a good test to see if it works