Fix prereqs

Added ruby.
LXC and KVM both need python-vm-builder.
And apt-cacher never worked for me, but apt-cacher-ng does.
Make using an invalid tag/commit an error
Merge pull request #21 from gavinandresen/prereq_fix

Fix prereqs
Merge pull request #22 from TheBlueMatt/master

Make using an invalid tag/commit an error
setarch on guest for LXC

Some packages use uname -m, which is not set to i386 on LXC on a 64 bit
host.

fixes #24
Do not hardcode suite in bootstrap fixup