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
diff --git a/.travis.yml b/.travis.yml
index 168482c..9092497 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,6 +7,10 @@
   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