calls to xcode tools should use xcrun

## motivation

This changes helps support development against different Xcode versions.

Suppose you are working on Xcode 6 support and you have used `xcode-select` to set the developer directory to `/Xcode/6b4/Xcode-Beta6.app/Contents/Developer`, but you want to check that your changes are backward compatible.

```
# test against Xcode 5.1.1
$ DEVELOPER_DIR=/Xcode/5.1.1/Xcode.app/Contents/Developer be rake spec
```

See $ man xcrun and $ man xcode-select for details about the DEVELOPER_DIR env var.