Build the documentation before pushing as well
Simplify Travis test suite

Do not test all syntax checkers anymore, because provisioning and
testing these takes far too long.  Instead, just check the core.

Should prevent core bugs, at least
Skip cfengine test if cfengine is missing
Merge branch 'refactor-travis'

* refactor-travis:
  Skip cfengine test if cfengine is missing
  Simplify Travis test suite