Add 8.0 to available iOS sdks. Fix Xcode path finder mechanism.

is there a way to do this w/out xcode-select?  The reason we didn't use xcode-select originally is because... you might be using a version of Xcode which is different than the one set via xcode-select.  e.g., suppose xcode-select points at /Applications/Xcode.app, but you're currently building with Xcode6-BlahBlah.app.  Things would break in that case.