docs: theme: transparency for favicon
travis-ci: enable apt caching (for speed)

This will tell Travis build machines to accept (network) local apt
caches for the launchpad PPA of gcc-arm-embedded, as per
http://docs.travis-ci.com/user/caching/#Caching-Ubuntu-packages
travis-ci: allow st_stm3220g_eval board to fail build tests

But build it anyways.