brew update for travis-ci on OSX
brew update for travis-ci on OSX

see https://github.com/Homebrew/brew/issues/3299

fplll/fplll,shi-bai/fplll,fplll/fplll,dstehle/fplll,shi-bai/fplll,cr-marcstevens/fplll,shi-bai/fplll,cr-marcstevens/fplll,dstehle/fplll,dstehle/fplll,cr-marcstevens/fplll,fplll/fplll,shi-bai/fplll,cr-marcstevens/fplll,fplll/fplll,fplll/fplll