travis: Run flake8 against Python 3.6

As with tox and make before it, we run everything that we can against
Python 3 (Python 3.6 in this case).

Signed-off-by: Stephen Finucane <stephen@that.guru>

.travis.yml

@@ -30,7 +30,7 @@ matrix:
       env: TOXENV=docs
     - python: '3.6'
       env: TOXENV=mypy
-    - python: '2.7'
+    - python: '3.6'
       env: TOXENV=flake8
 
 install: