Add continuous release deployment

[mgaruccio]

Right now releases are manual and there can be some lag between new code being merged and that code being available via npm. It would be nice if new pushes to master automatically generated a new release. It looks like Travis can support this, and I'd be happy to do the initial work to get things configured in the .travis.yml file as long as there's interest in getting the feature added/enabled.

Travis documentation on this - https://docs.travis-ci.com/user/deployment/releases/