Bring JDK current in Travis

[gliptak]

Gradle 6.0.1 breaks build.gradle "inheritance"

https://travis-ci.com/github/gliptak/jib/jobs/342161039#L492

#2503

[chanseokoh]

* Where:
Build file '/home/travis/build/gliptak/jib/jib-gradle-plugin/build.gradle' line: 2
* What went wrong:
An exception occurred applying plugin request [id: 'java-gradle-plugin']
> Failed to apply plugin [class 'org.gradle.api.plugins.JavaLibraryPlugin']
   > Cannot create variant 'classes' after configuration ':jib-gradle-plugin:apiElements' has been resolved

I think :apiElements is from the Java Library plugin (java-library). And the error seems to occur when applying java-gradle-plugin, whose doc says "[i]t automatically applies the Java Library plugin." The error message does seem to indicate that java-gradle-plugin is automatically applying java-library. But what's wrong? I'm not a Gradle expert, so not sure what's going on.

[chanseokoh]

I tried experimenting JDK 14. Unfortunately, we have integration tests where we test old Gradle versions (5.1 and 5.2, which is the minimum Gradle version we support), and these old versions do not run on JDK 13 or 14. I think building with JDK 13 or 14 should wait.

Thank you for trying this anyway!