Incorrect archive command

Previously, debs were handled by using the tar command to unarchive
them.  It turns out that while the version of tar on OS X and on
Ubuntu Raring can do this, the version on lucid (which is what
production is running on) cannot.  This change use the ar command
(which is what the deb spec declares) to extract the debs.  This
however, does not solve the current issue causing the buildpack to
fail, as the raring debs that are being downloaded depend on a
different version of libc than lucid provides.

[#49321993]