add flake8 to travis.yml

.travis.yml

 sudo: false
 language: python
-python:
-  - "2.6"
-  - "2.7"
-  - "3.3"
-  - "3.4"
-  - "3.5"
-  - "3.6"
-
-env:
-  - WTFORMS_VERSION=1
-  - WTFORMS_VERSION=2
+matrix:
+  include:
+    - python: 2.6
+      env: TOX_ENV=py26-WTForms1
+    - python: 2.6
+      env: TOX_ENV=py26-WTForms2
+    - python: 2.7
+      env: TOX_ENV=py27-WTForms1
+    - python: 2.7
+      env: TOX_ENV=py27-WTForms2
+    - python: 2.7
+      env: TOX_ENV=flake8
+    - python: 3.3
+      env: TOX_ENV=py33-WTForms1
+    - python: 3.3
+      env: TOX_ENV=py33-WTForms2
+    - python: 3.4
+      env: TOX_ENV=py34-WTForms1
+    - python: 3.4
+      env: TOX_ENV=py34-WTForms2
+    - python: 3.5
+      env: TOX_ENV=py35-WTForms1
+    - python: 3.5
+      env: TOX_ENV=py35-WTForms2
+    - python: 3.6
+      env: TOX_ENV=py36-WTForms1
+    - python: 3.6
+      env: TOX_ENV=py36-WTForms2
 
 addons:
   postgresql: "9.4"
@@ -27,7 +44,7 @@ before_script:
 install:
   - pip install tox
 
-script: tox -e py-WTForms$WTFORMS_VERSION
+script: tox -e $TOX_ENV
 
 after_success:
     - coveralls