Update to Java 12 and fix link in README to Oracle Java SE

[HEdingfield]

We should be linking to and using OpenJDK, per this article.

#239 is likely a blocker for this.

The most straightforward thing to link and use would probably be Oracle's open source builds of the JDK here.

[HEdingfield]

Counterpoint: we may want to stick with Java 11, since it's LTS until 2022 per the table here. In either case, we're still updating to a new version of Java.

[tarheel]

Any particular downside to going straight to 12?

[HEdingfield]

See my previous comment. Means we'd need to stay on top of new versions every 6 months, whereas 11 is LTS until 2022. It's probably still worth going to 12 and beyond, but we need to have a discussion about the certification process, who's responsible for building the code, if we need to stick with images frozen in time, etc.

[HEdingfield]

Addressed via #272. Now the tabulator only works in JDK 11+. When distributed using Gradle's distZip task, it requires that the user have the JDK installed. When distributed using Gradle's jlink task, it doesn't even require that the user have the JDK installed (the zip is 48 MB vs. 26 MB with distZip).