Travis CI: extended Python version coverage

.travis.yml

@@ -1,6 +1,11 @@
 language: python
 python:
+    - "2.6"
     - "2.7"
+    - "3.2"
+    - "3.3"
+    - "3.4"
+    - "3.5"
 sudo: false
 install: "pip install -r requirements.txt"
 script: nosetests