Travis CI: Look for unused imports

Modules that are imported but not used slow down our processing.
https://flake8.pycqa.org/en/latest/user/error-codes.html

.travis.yml

@@ -22,7 +22,7 @@ install:
 before_script:
   - echo "#before_script#"
   # stop the build if there are Python syntax errors or undefined names
-  - flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics
+  - flake8 . --count --select=F401,E9,F63,F72,F82 --show-source --statistics
   # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
   - flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
   - mkdir -p /tmp/walle/{logs,library,webroot,releases}