travis: Fix OSX build on travis

Run "brew update" before any installs.
This yields a clean build:
	https://travis-ci.org/williamtu/ovs-travis/builds/291616874

Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Gurucharan Shetty <guru@ovn.org>

.travis/osx-prepare.sh

@@ -3,4 +3,5 @@ set -ev
 pip install --user six
 pip install --user --upgrade docutils
 
+brew update || true
 brew uninstall libtool && brew install libtool || true