chore(tests): Also run tests on Python 2.6

tests/test_linter.py

@@ -15,8 +15,12 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import io
-
-import unittest
+import sys
+try:
+    assert sys.version_info >= (2, 7)
+    import unittest
+except:
+    import unittest2 as unittest
 
 from yamllint.config import YamlLintConfig
 from yamllint import linter