chore(travis): run two jobs

Instead of parallelization on a single Travis VM, we use two VMs.
- output is nicer (we don't have to buffer e2e tests and then show it at the end)
- you can easily see faster the result of unit tests (as it's basically a separate build)

We should also make sure we only do the necesary stuff (for install we don't need to do `grunt
package` for unit tests, we only need to generate the docs for e2e tests.