Allow failure of python 2.6

It really is not supported anymore by anyone, so it seems.

Author: Byron

.travis.yml

@@ -12,6 +12,7 @@ python:
   # - "pypy" - won't work as smmap doesn't work (see gitdb/.travis.yml for details)
 matrix:
   allow_failures:
+    - python: "2.6"
     - python: "3.6-dev"
     - python: "3.7-dev"
     - python: "nightly"