Dropped support for python 2.6 and added python 3.5

.travis.yml

 language: python
 sudo: false
 python:
-  - "2.6"
   - "2.7"
   - "3.3"
   - "3.4"
+  - "3.5"
 before_install:
   - pip install --find-links https://www.idiap.ch/software/bob/wheels/travis/ --use-wheel sphinx nose --pre coveralls
 install: