Fix Travis builds on macOS

Travis and Homebrew have been at odds: the Ruby in PATH is not what
Homebrew wants, so it prints:

 /usr/local/Homebrew/Library/Homebrew/brew.rb:12:in `<main>': Homebrew must be run under Ruby 2.3! You're running 2.0.0. (RuntimeError)

Info in the Travis's own GitHub bug report indicates "brew update" fixes
this issue, even though it increases the build time.

Signed-off-by: Thiago Macieira <[email protected]>

Author: thiagomacieira

.travis.yml

@@ -12,6 +12,7 @@ addons:
       - qt56base valgrind
 install:
   - if [ "${TRAVIS_OS_NAME}" != "linux" ]; then
+        brew update;
         brew install qt5;
     fi
 script: