cairo: make glib recommended.
brew-test-bot: add devel/HEAD install options.
libxml2: fix caveats typo.
Formula: temporarily set PYTHONPATH.

Needs a less hacky fix but at least this will stop failing tests.
Move getting formula names from bottles to method.
Warn more about failing bottle version detection.

This is needed for local bottle installation and also possible when
detecting bottle versions from URLs.
brew-test-bot: only force fetch with --cleanup.
brew-test-bot: only install bottle if created OK.