Travis: Work around xcode7.3 image bug

Refer to travis-ci/travis-ci#8552.  This was supposed to be fixed on
November 15, then on November 28.  Travis blew through both deadlines,
so I have no confidence that the issue will be fixed as promised in a
timely manner.  Adding 'brew update' to .travis.yml slows the OS X
build, but there is no choice at the moment.

.travis.yml

@@ -43,6 +43,7 @@ matrix:
 
 before_install:
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then
+      brew update &&
       brew install nasm homebrew/versions/gcc5 md5sha1sum Caskroom/versions/java6 &&
       ln -fs /usr/local/bin/gpg1 /usr/local/bin/gpg &&
       git clone --depth=1 https://github.com/libjpeg-turbo/gas-preprocessor.git ~/src/gas-preprocessor &&