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

This reverts commit 72710696.

This reverts commit d2722899.

This reverts commit a4e8ba3c.