README: Use distro packages, not CPAN

The XML::Simple and XML::SAX::Expat perl libraries are available as
Debian/Ubuntu packages.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
README: Remove requirement that /bin/sh is bash

We build fine with the default shell, and this change is pretty
intrusive.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Merge pull request #9 from jk-ozlabs/master

README: Use distro packages, not CPAN