update views & solution serializer

src/homework/migrations/0004_auto_20180204_1209.py

src/homework/migrations/0005_auto_20180204_1525.py

src/homework/migrations/0006_auto_20180204_1601.py

src/homework/permissions.py

src/homework/serializers.py

src/homework/urls.py

src/homework/views.py