Add argon2 to crypotgraphy step

- Add argon2-cffi
- Add django[argon2] option from Django docs:
  https://docs.djangoproject.com/en/1.10/topics/auth/passwords/#argon2-usage

This reverts commit 75870dd4, reversing
changes made to 0cf2e284.

Update setuptools

Preserve environment variables

Preserve LIBRARY_PATH and LD_LIBRARY_PATH.
Related heroku/heroku-buildpack-python#287

Update setuptools from 23.x.x to 25.x.x.

Switch pip-pop back to using pip 8

Fixes to the library vendoring's 'already exists' checks

Since if someone previously had `GDAL` in their requirements file, they
would already have the gdalserver binary present from `bin/steps/gdal`
but be missing the proj and geos vendor files. By checking for `proj`
instead, we ensure that the vendoring isn't incorrectly skipped in this
case.

For both the cedar and cedar-14 stacks, the libffi directory is named
`libffi-3.1` not `libffi-3.1.1`, so the existing library wouldn't be
detected, causing it to be re-downloaded during every compile.

Since we need to set it regardless of whether the vendored library
existed previously or not.

This means pip-pop will use the buildpack-installed pip 8, which doesn't
fail when encountering requirements files that use the new hash syntax.

Fixes #302.