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)

[dub357]

Looks like this is the only thing holding up #498
Any movement on this? The general public sure would like a new warbler version with all the fixes done in the last 7 years...