Revert "Remove Python 2.6 support."

This reverts commit 31d37a3953.

.travis.yml

@@ -17,6 +17,7 @@ env:
     - GOPATH=/tmp/go
     - PATH=$GOPATH/bin:$PATH
   matrix:
+    - TOXENV=py26 BOULDER_INTEGRATION=1
     - TOXENV=py27 BOULDER_INTEGRATION=1
     - TOXENV=lint
     - TOXENV=cover

setup.py

@@ -98,6 +98,7 @@ setup(
         'Operating System :: POSIX :: Linux',
         'Programming Language :: Python',
         'Programming Language :: Python :: 2',
+        'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Topic :: Internet :: WWW/HTTP',
         'Topic :: Security',

tox.ini

@@ -6,7 +6,7 @@
 # acme and letsencrypt are not yet on pypi, so when Tox invokes
 # "install *.zip", it will not find deps
 skipsdist = true
-envlist = py27,cover,lint
+envlist = py26,py27,py33,py34,cover,lint
 
 [testenv]
 commands =
@@ -40,6 +40,7 @@ commands =
     ./tox.cover.sh
 
 [testenv:lint]
+# recent versions of pylint do not support Python 2.6 (#97, #187)
 basepython = python2.7
 # separating into multiple invocations disables cross package
 # duplicate code checking; if one of the commands fails, others will