Publish dgraph-js with the next tag after a successful travis build

[gpahal]

On every successful travis build, we can publish dgraph-js to npm with the next tag.

Users can then install this bleeding edge version using npm install dgraph-js@next --save, test it and provide feedback.

[gpahal]

npm doesn't allow deleting already published versions. So the publish step will involve updating the version in the package.json to something like "1.0.5-beta.7" and creating a new version release which is not the best thing to do. So, this is not in the current plans.