Publish OpenWhisk standalone jar to Github release

To simplify usage of standalone openwhisk jar (#4516) for end users we should publish the jar under Github releases. This should follow an approach similar to what has been done for [wsk cli][1]

Few aspects

* The release should be done from master build
* It should make use of [multi stage builds](https://docs.travis-ci.com/user/deployment/releases/) as core repo as multiple steps involved
* The release should be marked under [pre-release](https://docs.travis-ci.com/user/deployment/releases/#advanced-options)
* Useful links
* [Conditional Releases](https://docs.travis-ci.com/user/deployment#conditional-releases-with-on)

[1]: https://github.com/apache/incubator-openwhisk-cli/releases