Bump to JDK11 for travis builds

[Khalian]

Update on JDK version for builds since 11 is the new LTS for OpenJDK.

[SethTisue]

adding 11 seems good, but I wouldn't suggest removing 8

[SethTisue]

@Khalian ping... interested in responding to the review feedback?

[Khalian]

Why keep 8 for travis?

[SethTisue]

In general, Scala projects build and test and publish on JDK 8. JDK 8 is the baseline version that Scala 2.12+ supports, and so almost everyone uses 8 in their CI to make sure that everything works on 8. Many projects also include 11 and/or 15 in their CIs, but as alternate testing paths, rather than as the main path.

However: this repo in particular is a Giter8 template, so arguably that alters things. The usual reason to publish on 8 is to ensure that the code doesn't accidentally include calls to JDK 11+ only API. But Giter8 templates aren't published as binaries, but as source.

Regardless: there is possibly some value in having CI ensure that everything works on 8. An example of a failure mode we might catch is that sometimes an upstream dependency accidentally publishes a binary that requires 11.

The reverse failure mode is less plausible, since Java stuff is almost always backward compatible, but it is possible for something to work on 8 but fail on 11, for example if the build passes an -XX flag that Oracle removed from 11, or if something runs afoul of the module system somehow, I suppose.

Summary:

• For a Giter8 template, it probably doesn't matter which JDK we use.
• Regardless, if we had to choose only one JDK to test on, I would choose 8.
• Regardless, why not test on both 8 and 11?

[Khalian]

Understood. Dropping the PR. Thanks.