Follow up with a rake build from a maintainer

[olleolleolle]

Background:

The .jar changes so rarely that it's not part of rake release to rake build the .jar.
But I think it's also a feature as it allows using Warbler from github: 'jruby/warbler' git.

For security reasons and to make sure compilation happens with the tools (Java version) intended it's best if it's performed by a maintainer (as a follow up of merging the PR).

Originally posted by @kares in #527 (review)