Ensure Java 1.8 is available on Darwin on CI

The build failed, and the logs say this:

  bazel: Java 1.8 is required to install this formula.
  Install AdoptOpenJDK 8 with Homebrew Cask:
    brew cask install homebrew/cask-versions/adoptopenjdk8
  Error: An unsatisfied requirement failed this build.

So let's try that then.

.travis.sh

@@ -20,6 +20,7 @@ case "$1" in
 			sudo dpkg -i bazel_1.1.0-linux-x86_64.deb
 			;;
 		    "osx")
+			brew cask install homebrew/cask-versions/adoptopenjdk8
 			brew install bazel
 			;;
 		esac