[.travis.yml] add flake8 linting of source files

Author: chrissimpkins

.travis.yml

@@ -4,18 +4,24 @@ sudo: false
 language: python
 matrix:
   include:
+    - python: 3.7
+      dist: xenial
+      env: TOX_ENV=lint
+      install: pip install flake8
+      script: flake8 --ignore=E501,W503 lib/fontline/*.py
     - python: 3.5
       env: TOX_ENV=py35
+      install: pip install tox
+      script: tox -e $TOX_ENV
     - python: 3.6
       env: TOX_ENV=py36
+      install: pip install tox
+      script: tox -e $TOX_ENV
     - python: 3.7
       env: TOX_ENV=py37
+      install: pip install tox
       dist: xenial
-
-script: tox -e $TOX_ENV
-
-install:
-    - pip install tox
+      script: tox -e $TOX_ENV
 
 notifications:
   email: false