Make Oracle JDK default for Debian distros

Without this instruction, the system continues to use the default OpenJDK.  This is confusing since we instruct the practitioner to install Oracle's JDK.

docs/INSTALLATION.md

    $ sudo add-apt-repository ppa:webupd8team/java
    $ sudo apt-get update
    $ sudo apt-get install oracle-java8-installer
+   $ sudo apt install oracle-java8-set-default
    ```
 -  Install Gradle: