Force Travis to use Node 0.11.13

The latest version of Node.js has a bug that
affects the Karma test runner.  A patch has been
merged to Karma, but has not landed in a version
yet.  Until a new version of Karma is released,
we should keep node at 0.11.13.

See karma-runner/karma#1182

(cherry picked from commit 9af2346a0cead634f3af5f390770ea65929c1f4a)