Travis CI: allow_failures in Python end of life branches

Related to the changes in #81
https://devguide.python.org/#status-of-python-branches vs.
https://devguide.python.org/devcycle/#end-of-life-branches

Author: cclauss

.travis.yml

@@ -7,6 +7,10 @@ jobs:
   fast_finish: true
   allow_failures:
     - env: TOXENV=docstrings
+    - env: TOXENV=py27
+    - env: TOXENV=py34
+    - env: TOXENV=py35
+    - env: TOXENV=pypy
   include:
     - python: 2.7
       env: TOXENV=py27