chore(travis): run e2e tests as a separate job

Closes #1222
fix(introspection): getTestability should throw when ElementProbes are unavailable
feat(testability): whenStable replaces notifyWhenNoOutstandingRequests

BREAKING CHANGE:
  NOTE: This only affects you if you are calling this API directly.  If
  you are using ProtractorDart, then you are insulated from this change.

  To update your code, rename all references to the
  notifyWhenNoOutstandingRequests(callback) method on the testability
  object to whenStable(callback).
feat(urls): support relative CSS / template URLs in components