Add OpenJDK 8 for Java based builds

[jehrhardt]

Currently a Java based build can only choose between OpenJDK 6 and 7 and Oracle JDK 7 and 8. The option OpenJDK 8 is missing.

[BanzaiMan]

See travis-ci/apt-source-safelist#177

[omajid]

As an open source developer, I care more about my projects working with OpenJDK 8 than I do about them working with a proprietary JDK version.

Is there anything I can do to improve the support for OpenJDK (6, 7, 8 and 9) in travis-ci?

[grahamtriggs]

My project is now in a situation where I'm going to have to ditch support for Java 7.

However, I don't want to ditch support for OpenJDK. Anything that can be done to bring latest version OpenJDK support to Travis would be a really big benefit.

[aaime]

Same here, geoserver.org now builds only on Java 8 and we're missing a way to test openjdk.
There are some subtle differences, for example in graphics2d, that make certain rendering test pass on Oracle and fail on OpenJDK, we would like to know that before merging pull requests :-)

[rklaehn]

Is this still not possible? We really need this.

[mcamou]

Given Oracle's recent stance on JDK licensing, I think this should be a priority.