Fix JitPack build (#677 / #678)

From the
[JitPack docs](https://jitpack.io/docs/BUILDING/#java-version):

>JitPack will compile projects using OpenJDK Java 8. See the example
>projects on how to set a different target version in your build file.
>
>Maven projects that specify a target version in their pom will be
>built with that target version.
>
>If your project uses Travis or Circle CI then JitPack will read the
>lowest jdk version from yml file and use that to build.
>
>Alternatively create a jitpack.yml file in the root of your
>repository and specify a jdk version:

This PR adds a `jitpack.yml`, specifying OpenJDK 11 to be used.

Closes: #677
PR: #678

jitpack.yml

@@ -0,0 +1,2 @@
+jdk:
+ - openjdk11