Improve flake8 config

- add flake8 config file (max line length now is 90 chars)
    [http://katafrakt.me/2017/09/16/80-characters-line-length-limit/]
- simplify flake8 call in travis config

.flake8

@@ -0,0 +1,4 @@
+[flake8]
+max-line-length = 90
+max-complexity = 10
+exclude = *migrations*

.travis.yml

@@ -17,7 +17,7 @@ matrix:
       dist: xenial
       sudo: true
 before_script:
-  - flake8 --exclude=*migrations* knox knox_project tests
+  - flake8 .
 script:
   - tox -e $TOX_ENVS
 deploy: