Remove py26 oldest tests (#4856)

* Remove py26 oldest tests.

The only systems where we support Python 2.6 use certbot-auto so the oldest
supported versions of our dependencies are never used when using supported
installation methods. Let's remove this unnecessary and slow test.

* Make tox.ini happy

* Remove py26-oldest from Travis

.travis.yml

@@ -33,13 +33,6 @@ matrix:
         - sudo cat /var/log/mysql/error.log
         - ps aux | grep mysql
       services: docker
-    - python: "2.6"
-      env: TOXENV=py26-oldest BOULDER_INTEGRATION=1
-      sudo: required
-      after_failure:
-        - sudo cat /var/log/mysql/error.log
-        - ps aux | grep mysql
-      services: docker
     - python: "2.7"
       env: TOXENV=py27_install BOULDER_INTEGRATION=1
       sudo: required