chore(travis): speed up the build

Just wanna see the travis build...

- parallelize the tasks
- cache requests (e2e tests)

This reduces the time from ~18min to ~12min.

It makes the output little messy. We could buffer output of each task and display it once it's fully finished, nicely. I think giving instant feedback is better.

Conflicts:
	Gruntfile.js
	package.json