multirust is slow on OS X

After installing multirust on my MacBook Pro (OS X 10.11 beta), I noticed that getting from typing rustc to getting the help screen felt more sluggish than usual. To test this, I changed the last line of `multirustproxy` to just echo the command rather than running it, and timed it; it took between 100 and 200ms, which is quite a lot for a tool whose purpose is just to locate a toolchain.

Once I realized it was entirely a shell script, I tried doing the same in my Linux VM - 15ms. Welp.

Switching the scripts from `/bin/sh` (which is `bash` on OS X) to my Homebrew-installed `dash` brought the time down a bit to be often in the 80-90ms range, but that's not much of a difference.

Unless there's some factor I'm missing, I can only conclude that OS X is slow at process creation and thus shell scripts. Not exactly multirust's fault, but if you ever consider rewriting it in Rust or something, know that it would improve the user experience on OS X. :)