Run flake8 to find syntax errors and undefined names (#439)

__E901,E999,F821,F822,F823__ are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. The other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety.  This PR therefore recommends a flake8 run of those tests on the entire codebase.
* F821: undefined name `name`
* F822: undefined name `name` in `__all__`
* F823: local variable `name` referenced before assignment
* E901: SyntaxError or IndentationError
* E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

.travis.yml

@@ -10,5 +10,5 @@ install:
     - docker build . -t baselines-test
 
 script:
-    - flake8 .
+    - flake8 . --show-source --statistics
     - docker run baselines-test pytest -v .