Update to new form of brew cask install command

Author: mfitton

.travis.yml

@@ -219,7 +219,7 @@ matrix:
         - . ./ci/travis/ci.sh init RAY_CI_MACOS_WHEELS_AFFECTED,RAY_CI_JAVA_AFFECTED,RAY_CI_STREAMING_JAVA_AFFECTED
       before_script:
         - brew tap adoptopenjdk/openjdk
-        - brew cask install adoptopenjdk8
+        - brew install --cask adoptopenjdk8
         - export JAVA_HOME=/Library/Java/JavaVirtualMachines/adoptopenjdk-8.jdk/Contents/Home
         - java -version
         - . ./ci/travis/ci.sh build