Allow specific version of Debian package

Hey Ronan,

This PR allows `require.package` (as well as `deb.install`) to specify a version, for ex:

	def install_specific_packages():
	    require.deb.package('firefox', version='11.0+build1-0ubuntu4')
	    deb.install('emacs', version='23.3+1-1ubuntu9')

Merci!

@cgphillips contributed to this PR as well ;)