Use RbConfig rather than Config

RbConfig and Config are both names for the compile-time configuration
constants out of Ruby as far back as 1.8.5, so we can happily use the newer
name.

This eliminates a deprecation warning from Ruby 1.9, which finally started
complaining that we should have made this move already.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>