Make min and pinned requirement proper release time generated files

These files are now no longer stored in the repo and are to be built at release
time.  So no longer any confusion about changing these files during
normal development.

Hopefully this opens up some more testing against current and latest for
us.

The only issue I can see is that we don't have a potential mechanism to discover new releases of dependencies.  Thus we don't know if e.g. ``django-taggit`` changes version from v1.1 to v1.2 during development.  We don't have that at the moment but we could have done a diff and failed a build if our stored pinned requirements was different to a newly generated pinned requirements, indicating that things have changed and need review.