Split requirements based on the environment

[webknjaz]

Create base-requirements.txt:

dj-database-url==0.4.2
Django==1.8

and 'require' that file right in requirements.txt using -r requirements/base.txt, so it would look like:

-r requirements/base.txt
gunicorn==19.6.0
psycopg2==2.6.2
whitenoise==3.2.3

also you may want to add requirements/test.txt

-r requirements/base.txt
pre-commit

After this is complete, you'll be able to use pip install -r requirements/test.txt in Travis (do this!), pip install -r requirements/base.txt for setting up bare env suitable and Heroku will keep picking up same requirements as earlier (it fact, it just runs pip install -r requirements/base.txt in a nutshell once it detects that it's a Python app).