require at least one commit check

[travi]

if no commit checks are added, the list of checks is considered "all passing". this is an issue with travis is registered, but builds are disabled because the checks dont get added.

since the list is considered all passing greenkeeper-keeper happily merges the PR, even tho no checks were evaluated

[travi]

one check in addition to the greenkeeper/verify check that exists for every greenkeeper PR

[travi]

its probably better to compare against those registered for the repo: https://developer.github.com/v3/repos/hooks/#list-hooks