branch build when no github token?

[Allen Wittenauer]

Some CI systems (e.g., Circle CI) does not automatically provide a Github 
token.  Would it make sense to switch to branch builds when:

* The build is determined to be from a PR
* The PR pull fails

Obviously a warning should be given to tell the user _why_ it switched. For 
existing users this change might be unexpected but overall it feels more 
‘friendly’ to do as it could hit that “Yetus is fine; your setup is not.”  I 
know it took _me_ a bit to realize what was wrong.