Add Flake8 F4 Tests to .travis.yml

F4 tests check for import errors. Implements issue TheAlgorithms#973

Author: PatOnTheBack

.travis.yml

@@ -2,4 +2,4 @@ language: python
 dist: xenial  # required for Python >= 3.7
 python: 3.7
 install: pip install flake8
-script: flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
+script: flake8 . --count --select=E9,F4,F63,F7,F82 --show-source --statistics