Remove travis-after-all (deprecated)

[coliff]

The Gulp file currently has 'travis-after-all' though this project has been deprecated (https://github.com/alrra/travis-after-all).
The functionality it provided is now supported natively by Travis CI via Build Stages (https://blog.travis-ci.com/2017-05-11-introducing-build-stages).

I'm not too familiar with Travis so opening this as an issue if someone else wants to open a PR

[reddysridhar53]

Is this still open?

[roblarsen]

@reddysridhar53 It is