Bug 1107523 - Run flake8 as part of the Travis build

flake8 is pyflakes+pep8. In a later PR I'll add a mention of it to the
docs - particularly how to set it up as a local git commit hook, but for
now I'm just keen to not regress the passing flake8 run. We may also
need to further tweak the ignore settings in setup.cfg if we find
certain warning types to be too annoying.

.travis.yml

@@ -31,6 +31,7 @@ before_script:
   - python setup.py build_ext --inplace
   - mysql -e 'create database treeherder;'
 script:
+  - flake8 --show-source
   - py.test tests/$* --runslow
 notifications:
   email: false

requirements/dev.txt

@@ -22,3 +22,5 @@ virtualenv==1.7.1.2
 
 #for celery auto-reloading
 pyinotify==0.9.4
+
+flake8