restrict tox to python2.7

mox isn't python3 ready (syntax error) thus restrict to 2

tox.ini

+[tox]
+envlist=py27
+
 [testenv]
 deps=
   nose
@@ -5,6 +8,7 @@ deps=
   coverage
 commands=
   nosetests -v \
-	--with-coverage --cover-package=cam --cover-erase --cover-html --cover-html-dir=htmlcov \
+    --with-coverage --cover-package=cam --cover-erase \
+    --cover-html --cover-html-dir=htmlcov \
         []        # substitute with tox' positional arguments