Merge pull request #221 from exercism/debian-oracle-jdk-default

Make Oracle JDK default for Debian distros

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: