chibi-scheme: 0.7 -> 0.7.3

Update the `chibi-scheme` (attribute `chibi`) package from version 0.7
to version 0.7.3.

The homepage listed for this package before this change,
<https://code.google.com/p/chibi-scheme/>, now redirects to
<https://github.com/ashinn/chibi-scheme>; this patch changes this
package to use this GitHub version of the software.

I have tested this change per nixpkgs manual section 10.1 ("Making
patches").