Travis: Drop Python 2.6

Markdown keeps breaking the 2.6 build. We could pin an earlier version,
or we can simply drop 2.6 support. The Python world is moving on; time
for us to do the same.

.travis.yml

@@ -1,6 +1,5 @@
 language: python
 python:
-    - "2.6"
     - "2.7"
     - "3.4"
     - "3.5"