Updating project to run on Dart 1.4

This requires updating barback dependency to 0.13.0, following steps from https://groups.google.com/a/dartlang.org/forum/#!topic/misc/579fz2SRnLA to maintain compatibility with Dart 1.3.
Merge pull request #27 from blois/barback_0.13.0

Updating project to run on Dart 1.4