Define a minimum Scrapy version

[Gallaecio]

We should indicate a minimum Scrapy version in the README.

I also wonder if Scrapy should be a dependency, setup.py-wise, so that the right version of Scrapy (or, more likely, the other way around) can be enforced during installation. scrapy-zyte-smartproxy does it.

[BurnzZ]

I think we could perhaps set 2.6, as the minimum as briefly discussed in #10 (comment).

+1 on enforcing it in the install_requires param.