Fix hard coded path that does not honour xcode-select in builder.
Fix hard coded path that does not honour xcode-select in builder.

This is causing problems building amd64 iOS 7.1 using Xcode5.1-Beta3 as it is not in the usual path.

katsuyoshi/RubyMotion,beni55/RubyMotion,katsuyoshi/RubyMotion,incbee/RubyMotion,incbee/RubyMotion,jjaffeux/RubyMotion,jjaffeux/RubyMotion,hboon/RubyMotion,incbee/RubyMotion,HipByte/RubyMotion,katsuyoshi/RubyMotion,HipByte/RubyMotion,hboon/RubyMotion,jjaffeux/RubyMotion,beni55/RubyMotion,hboon/RubyMotion,beni55/RubyMotion,HipByte/RubyMotion,jjaffeux/RubyMotion,incbee/RubyMotion,beni55/RubyMotion,katsuyoshi/RubyMotion