Explicitly define all supported python environment

With only 3.5, travis-ci works, because 2.7, 3.3 and 3.4 interpreters
are present by default,  but it might break randomly at some point.

Definining all the versions explicitly, and using the tox-travis plugin
is more robust.

Also, 2.6 was removed from the "officially supported" python versions,
as the tests are not run (and don't pass).

.travis.yml

@@ -1,9 +1,12 @@
 language: python
 python:
+  - "2.7"
+  - "3.3"
+  - "3.4"
   - "3.5"
 sudo: false
 install:
-  - pip install tox
+  - pip install tox-travis
 script:
   - tox
 services:

README.rst

@@ -33,7 +33,7 @@ Requirements
 ------------
 
 * Redis.
-* Python 2.6, 2.7 or 3.3+.
+* Python 2.7 or 3.3+.
 
 Installation
 ------------

setup.py

@@ -26,7 +26,6 @@ setup(
         'Operating System :: OS Independent',
         'Programming Language :: Python',
         'Programming Language :: Python :: 2',
-        'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.3',