chore(gulp): don't run pub get in parallel to avoid a race condition

The build fails very often, see https://travis-ci.org/angular/angular/pull_requests

This is something related to [`pub get`](https://travis-ci.org/angular/angular/builds/43000576#L440). I suspect a race condition in the pub cache.

Running the `pub get` commands sequentially would avoid this from happening. The build time is impacted but I have to re-launch every single PR 2/3 times before they get through due to this error.

@tbosch what do you think ?