Travis-CI: add Python 3, with failures enabled (for now)

The goal of this change is to inspect how Python 3 compatibility plays
in the Travis-CI environment, while not blocking the supported Python
2 builds.

IIUC, when using "system_site_packages", we're stuck with the Python
version on Ubuntu Trusty (2.7.6 and 3.4.3).  That's the reason behind
the specific "3.4" version added there.

[1] https://docs.travis-ci.com/user/languages/python/#Travis-CI-Uses-Isolated-virtualenvsSigned-off-by: NCleber Rosa <crosa@redhat.com>

.travis.yml

@@ -5,6 +5,11 @@ virtualenv:
 
 python:
     - "2.7"
+    - "3.4"
+
+matrix:
+  allow_failures:
+    - python: "3.4"
 
 branches:
     only: