Use the same interpreter for the shims that is also used to run the main
Homebrew process (the one invoked via `brew`). The magic basically lies
in executing `ruby` with the `-x` option (supported since at least 1.8)
and in the following shebang line.
This strategy is only used in one formula, so we can just replace it
with a dependency. It could be done with a requirement, but since this
is a low-impact change (it only matters for head builds on Leopard), I'm
not sure it's worth the effort.
The constant is retained for compatibility, though I could find only one
other reference to it on GitHub, in a formula that was removed from core
almost two years ago.
