feat(travis): Only submit if all the builds complete

Adds https://github.com/dmakhno/travis_after_all trick to AngularDart.

This solution works, but isn't terribly well-engineered.

(a) I didn't want to copy @dmarkhno's code into our repo, so we are referencing a fork under my name.
(b) A more elegant solution would be to rewrite this code in Node and publish it to npm.  Then we could just say:

```
after_success:
- if [ $(node node_modules/travis_all/all_success) ]; then ./scripts/publish; fi
```

For now, though, this works and makes the presubmit much more valuable.