Use Travis stages to run JS tests separately

[ArthurHoaro]

Related to this comment.

I chose to use stages here instead of matrix because, in my understanding, using matrix would require a bunch of if in the script section.

The only downside I see here is that the JS tests don't run in parallel, they wait for the test stage to be successfully completed.

[virtualtam]

Interesting resources for configuring Travis CI with projects/repositories using multiple programming languages:

• https://stackoverflow.com/questions/27644586/how-to-set-up-travis-ci-with-multiple-languages#44054333
• https://github.com/firebase/firebase-ios-sdk/blob/master/.travis.yml

I didn't know the build matrix could be used in such a way... let me know if I should take a look at this to propose a future PR :)

[ArthurHoaro]

Thank you for you resources. It works surprisingly well because the directives inside the matrix (:neo:) seems to override the global ones.

[virtualtam]