Add python 2.6 to travis, allow failure. See if we can try to cover it

Author: tony

.travis.yml

@@ -1,8 +1,12 @@
 language: python
 
 python:
-  - "2.7"
-  - "3.3"
+  - 2.6
+  - 2.7
+  - 3.3
+matrix:
+  allow_failures:
+    - python: 2.6
 env:
   - TMUX_VERSION=master
   - TMUX_VERSION=1.8