Fix brew runtime error on travis
Fix brew runtime error on travis

Homebrew requires Ruby 2.3 to run.
The Travis images for OSX are outdated,
and the only way to get Homebrew working is
to run brew update before doing anything else,
as suggested in
https://github.com/Homebrew/brew/issues/3299

sociomantic/libdrizzle-redux,sociomantic/libdrizzle-redux,sociomantic/libdrizzle-redux