pin versions for Squeeze and Wheezy, also remove py26 from default run

Python2.6 is EOL and my distributions is about to remove it. Travis CI
uses Ubuntu 12.04 LTS and can still test Python2.6 for at least three
years.

.travis.yml

@@ -5,10 +5,11 @@ env:
   - TOX_ENV=py27
   - TOX_ENV=py33
   - TOX_ENV=py34
-  - TOX_ENV=backport
+  - TOX_ENV=squeeze
+  - TOX_ENV=wheezy
 matrix:
   allow_failures:
-    - env: TOX_ENV=backport
+    - env: TOX_ENV=squeeze
 install:
   - pip install tox
   - sudo rm -rf /dev/shm && sudo ln -s /run/shm /dev/shm

tox.ini

@@ -1,28 +1,40 @@
 [tox]
-envlist = py26,py27,py33,py34
+envlist = py27,py33,py34
+
+[testenv]
+deps =
+    nose
+commands =
+    python setup.py nosetests
+
 [testenv:py26]
 deps =
     argparse
-    configparser
     unittest2
+    configparser
     {[testenv]deps}
+
 [testenv:py27]
 deps =
     configparser
     {[testenv]deps}
-[testenv:backport]
-deps =
-    configparser
+
+[testenv:debian]
+deps=
     werkzeug==0.8.3
     html5lib==0.95
     itsdangerous==0.22
     ipaddr==2.1.10
     misaka==1.0.2
-    {[testenv]deps}
-[testenv]
+
+[testenv:squeeze]
+basepython=python2.6
+deps=
+    {[testenv:py26]deps}
+    {[testenv:debian]deps}
+
+[testenv:wheezy]
+basepython=python2.7
 deps =
-    nose
-commands =
-    python setup.py nosetests
-install_command =
-    pip install --allow-external ipaddr --allow-unverified ipaddr {opts} {packages}
+    {[testenv:py27]deps}
+    {[testenv:debian]deps}