Secret for publishing signed jars must be updated

Following the recent [sbt upgrade](https://github.com/combust/mleap/pull/818), the "publish signed jars" step [broke](https://app.travis-ci.com/github/combust/mleap/jobs/575823488) cause it relies on an encrypted file `travis/travis.tar` extracted in `travis/extract.sh` which has some code in it that still relies on the old sbt. We need to either:

- someone with the right key/iv/passphrase to decrypt the file to fix it
- overhaul the publishing mechanism such that we don't keep encrypted secrets in the repo.

I haven't researched how to do the latter (and I may not have the time/resources), but it seems preferable in the long-term and I assume other open sources projects have figured it out.

We won't be able to make a new MLeap release until this is fixed.

Start by reading travis/extract.sh and inspecting the encrypted travis/travis.tar used by the “publish signed jars” Travis job, then compare that path with the recent sbt upgrade in PR 818. Done means the publish-signed-jars step succeeds with an updated publishing mechanism or corrected encrypted artifact, allowing a new MLeap release.

Written by the indexing model from the issue text.