xctool: updating to 0.1.7

Upgrade to 0.1.7 and switch to using the binary releases.

People can still install from source if they use --HEAD, but the
binary releases are preferred since they've built in such a way that
they're compatible with older versions of the iOS SDK (going back to
5.0).

0.1.7 also introduces a new dir layout where the xctool files are stored
in bin, lib, and libexec dirs -- https://github.com/facebook/xctool/commit/d6592a7

The old formula will probably break for anyone that tries to use --HEAD :-(