Add declarative widgets jar to release

[poplav]

Fixes #455:

• Utilizes the sbt-sonatype plugin
• The env variables added to travis are:
  • encrypted_9067f25a9380_key, encrypted_9067f25a9380_iv used in the travis before_install target to decrypt the encrypted secret file hosted in github
  • PGP_PASSPHRASE passphrase for the secret once it is decrypted
  • REPO_USERNAME, REPO_PASSWORD nexus username/password
• When issuing a new release the only change would be to modify the version := variable in kernel-scala/build.sbt
• Can search/view the packages released/hosted on search.maven.org
• Tested and can utilize in a notebook e.g. %AddDeps org.jupyter declarativewidgets_2.10 0.6.1_dev --transitive --trace --verbose
• Todos
  • Still need to update the release process wiki. Was holding off to document after an official release, as the testing that was done so far was replacing make all with make publish-scala-jar in travis.
  • Comment of the sonatype issue that a release was created.
  • Assuming we can leave example scala notebook as is.
  • Update the readme to specific release info or just point to search.maven.org

[lbustelo]

We are going to need a way to set this version based on the version in https://github.com/jupyter-incubator/declarativewidgets/blob/master/nb-extension/python/urth/widgets/ext/_version.py.

[poplav]

@lbustelo updated the PR. publish jar depends on export release. It uses a file as the intermediary between the two targets. I was having an issue exporting as it was operating in a different shell. Maybe we should treat the version setting differently to avoid this commit.

[lbustelo]

I think this is missing a dependency... for it to be a standalone, shouldn't it depend on the recipe that builds scala

[poplav]

added dependency with the latest commit