Using deps from `install_requires` if `requirements.txt` is not declared

[BurnzZ]

If the following isn't declared in scrapinghub.yml:

requirements:
  file: requirements.txt

It might be good to use the dependencies declared in install_requires inside the setup.py file.

However, unlike in #123, one workaround we could do is create a temporary requirements file during spider deployment to get around the eggs issue (if it's not fixed yet in the backend).