Strip signatures from .jar file

Until we properly sign the .jar file, those signatures just result in a
`SecurityException`, making the .jar file unable to run. I got this fix
by

* https://github.com/theapache64/stackzy/commit/9dc0f64d79de32786fa6250bdf4b28416ab38eee
* https://github.com/theapache64/stackzy/issues/41

Fixes #119.