Make flake8 ignore parse errors in docs directory

The docs directory is currently parsed by flake8 causing build
errors. This change just makes the docs dir ignored for travis
flake8 tests.

.travis.yml

@@ -17,7 +17,7 @@ script:
     set -e -o pipefail;
     if [ "$TRAVIS_PULL_REQUEST" != "false" ]
       then git diff $TRAVIS_COMMIT_RANGE | flake8 --diff;
-      else git fetch -q origin master:origin/master && git diff origin/master... | flake8 --diff
+      else git fetch -q origin master:origin/master && git diff origin/master... !(docs) | flake8 --diff
     fi
 
   # Then run the tests