"Use RbConfig instead of obsolete and deprecated Config."

[Meowyam]

I get this echoed to stderr everytime I use this module from something else.

I feel like it's making me look bad. Switching Config to RbConfig fixes it and it still works on both 1.8 and 1.9 but without the warning.

[Ben Oakes]

This is fixed by #3.

[Ben Oakes]

This issue was fixed when #3 was merged. It can be closed.

/cc @trans