CI: test python 3.4-3.7; lint with flake8 (#191)

.travis.yml

@@ -1,6 +1,9 @@
-dist: xenial   # required for Python >= 3.7
+dist: xenial
 language: python
 python:
+  - "3.4"
+  - "3.5"
+  - "3.6"
   - "3.7"
 install:
   - pip install -e './[dev]'
@@ -9,3 +12,4 @@ before_script:
 # command to run tests
 script:
   - python testproject/manage.py test coderedcms --settings=coderedcms.tests.settings
+  - flake8 coderedcms testproject

setup.cfg

@@ -0,0 +1,3 @@
+[flake8]
+exclude = coderedcms/project_template/*,migrations
+max-line-length = 120

setup.py

@@ -55,7 +55,7 @@ setup(
     extras_require={
         'dev': [
             'libsass',
-            'pylint-django',
+            'flake8',
             'sphinx',
             'twine',
             'wheel'