Add flake8 to travis

.flake8

@@ -0,0 +1,3 @@
+[flake8]
+max-line-length = 120
+ignore=C408,C815,A003

.travis.yml

@@ -10,6 +10,9 @@ python:
   - '3.6'
 
 install:
-  - pip install -U pip setuptools pytest
+  - pip install -U pip setuptools
+  - pip install -U pytest flake8-tuple flake8-quotes flake8-comprehensions flake8-commas flake8-builtins
 
-script: 'python setup.py test'
+script:
+  - python setup.py test
+  - flake8 disco/