Improve testing (#415)

These changes:

 - Fix the lint and the djmaster jobs
 - Fix compatibility with upcoming Django 3.0
 - Replace jobs with matrix in .travis.yml
 - Add a test job for Python 3.8
 - Allow running tests on any branch

testproj/snippets/serializers.py

@@ -13,7 +13,6 @@ else:
     from django.core.validators import MaxLengthValidator, MinValueValidator
 
 
-
 class LanguageSerializer(serializers.Serializer):
     name = serializers.ChoiceField(
         choices=LANGUAGE_CHOICES, default='python', help_text='The name of the programming language')