Allow Travis run to fail on Python 3

I'll file a separate issue for adding Python 3 support.

Author: edmorley

.travis.yml

@@ -14,3 +14,9 @@ script:
   # TODO: Replace with an actual test suite:
   # https://github.com/kennethreitz/bob-builder/issues/31
   - bob --help
+matrix:
+  allow_failures:
+    - python: "3.4"
+    - python: "3.5"
+    - python: "3.6"
+  fast_finish: true