Compliance with OEP-18

Update setup.py install_requirements definition.
Update travis file using new requirements.
Update tox file using new requirements.
Create requirements folder.
create pip_tools, base, test, travis and tox pip-tools requirements files.
Create Makefile and add upgrade command.
Include requirements files generated using upgrade command.
Update openedx.yml OEPs list.
Add venv to gitignore.