Drop default support for .travis.yml but still leave it as an option for integrators

[samrocketman]

Previously Jervis would search for the following YAML files: .jervis.yml falling back to .travis.yml.

Jervis has long-outgrown Travis YAML. This issue is to remove all references of .travis.yml.

New YAML standard

The overwhelming majority of the community writing YAML now prefers the .yaml extension. While I personally prefer .yml I think it makes sense to prioritize the community norm.

Jervis will now search for the following YAML files: .jervis.yaml falling back to .jervis.yml.