WFE-1273 Relative URI within zip-file
WFE-1273 Use URI.route_to for relative_uri

Old code could mistakenly give out "." for URIs that didn't end in /,
and would also try to relativize based on string-prefix alone
(http://example.com/fred2/file.txt relative to http://example.com/fred
is not "2/file.txt" :) )
Gemfile.lock needed to avoid surprises on new machines
Correct Ruby syntax
Return unchanged if already relative
Use app:// URIs if context is relative
Updated schema
Correct block syntax w/argument
Do not relativize above ro_uri -- URI of the RO
Merge branch 'relative-uri' into packs