Change the default script for Objective-C to xctool

This changes the default build script for Objective-C build to use xctool if a valid configuration is passed.

I'm not completely sure about the config variable names. If we prefix them with `xcode_` we'll maintain backwards compatibility, although the old variables were never documented. I should probably check if anyone is using them.

TODO:

- [x] Print a warning if the config is invalid. That is, the scheme or either the workspace or project is missing.
- [x] Test on staging.

To the people I'm about to ping to ask to do a quick sanity check on this:

This change makes `xctool` the default script if a scheme is specified in the `.travis.yml` file, like this:

``` YAML
language: objective-c
workspace: YourWorkspace.xcworkspace
scheme: YourScheme
```

Does it seem good to require workspace/project and scheme? Can one or more be left out, or should I include more? There's also `xctool_args`, which can be added to pass arbritary args to xctool, like `-sdk`.

The above config will run this command:

    $ xctool -workspace YourWorkspace.xcworkspace -scheme YourScheme build test

Should the default command be something else?