use flake8 instead of pep8 in travis build

.travis.yml

@@ -4,10 +4,10 @@ python:
     - "2.7"
 
 install:
-    pip install mock coverage coveralls pep8
+    pip install mock coverage coveralls flake8
 
 before_script:
-    pep8 mozregression tests setup.py
+    flake8 mozregression tests setup.py
 
 script:
     coverage run setup.py test