Update the document regarding Open JDK

Since we have switched to use Open JDK, our documents need to be changed to point
to Open JDK instead of Oracle JDK.

Partially-closes: apache/openwhisk-release#169

tools/macos/README.md

@@ -27,25 +27,23 @@ If you prefer to use Docker-machine, you can follow instructions in [docker-mach
 The following are required to build and deploy OpenWhisk from a Mac host:
 
 - [Docker 1.12.0](https://docs.docker.com/docker-for-mac/)
-- [Java 8](http://www.oracle.com/technetwork/java/javase/downloads/index.html)
+- [Open JDK 8](https://adoptopenjdk.net/releases.html#x64_mac)
 - [Scala 2.11](http://scala-lang.org/download/)
 - [Ansible 2.5.2](http://docs.ansible.com/ansible/intro_installation.html)
 
 **Tip** Versions of Docker and Ansible are lower than the latest released versions, the versions used in OpenWhisk are pinned to have stability during continuous integration and deployment.
 
 
-[Homebrew](http://brew.sh/) is an easy way to install all of these and prepare your Mac to build and deploy OpenWhisk. The following shell command is provided for your convenience to install `brew` with [Cask](https://github.com/caskroom/homebrew-cask) and bootstraps these to complete the setup. Copy the entire section below and paste it into your terminal to run it.
+[Homebrew](http://brew.sh/) is an easy way to install all of these and prepare your Mac to build and deploy OpenWhisk. Copy the entire section below and paste it into your terminal to run it.
 
 ```
 echo '
 # install homebrew
 /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
-# install cask
-brew tap caskroom/cask
-# install for finding alternative versions (java8)
-brew tap caskroom/versions
-# install java 8
-brew cask install java8
+# install for finding alternative versions (Open JDK 8)
+brew tap AdoptOpenJDK/openjdk
+# install Open JDK 8
+brew install adoptopenjdk-openjdk8
 # install scala
 brew install scala
 # install pip