(maint) Remove stale files from ext/rack

The ext/rack directory had an outdated README file and an ancient manifest.pp
file, and the useful files (config.ru and the example vhost) were buried in a
subdirectory.

This commit removes the old files, moves the good files up a level, and adds a
comment to the vhost pointing to the maintained passenger documentation.
(maint) Update debian packaging to reference new rack support location

As updated in commit 4c9b63f, the sample files for rack passenger deployment
have been moved up a directory. The puppetmaster-passenger package for debian
uses those files, so this commit updates the debian/rules file with their new
location. It also updates the README_DEVELOPER.md, which references the same
files.
Merge pull request #1951 from nfagerlund/stale_rack_files

(maint) Remove stale files from ext/rack