Update .travis.yml

The haxm-darwin Travis build has been failing due to HomeBrew version is out of date. So add the update step to the build script.
Error: Your Homebrew is outdated. Please run `brew update`.
Error: Kernel.exit

Author: wayne-ma

.travis.yml

@@ -8,6 +8,7 @@ matrix:
         homebrew:
           packages:
             - nasm
+          update: true
       script:
         - cd platforms/darwin
         - xcodebuild -configuration Debug -sdk macosx