Correct typos and non-canonical/HTTPS URLs

https://devcenter.heroku.com/articles/buildpacks#officially-supported-buildpacks

To save the 301s.

Fix #277 - Install libffi with django[bcrypt] req.

This ensures that libffi, required by bcrypt, is installed when Django
is installed with the bcrypt dependency declared as an extra
(django[bcrypt] or Django[bcrypt]).

Correct buildpack featured image

Closes #275

Fixed some broken comments