Add option to use xcodebuild instead of xctool.

[jmromer]

Hi @burtlo 👋

I came across your gem while working on the Exercism.io Objective-C track.

Unfortunately xctool doesn't support the latest version of Xcode, so I made this modification to allow using xcodebuild instead.

Let me know if you think this would be useful in the published gem.

Sample output using Xcode 8.3.1 (some noise removed):

% objc HelloWorld

Looking for all files with prefix: HelloWorld
Loading files into ExercismTestFixture (Project) at /var/folders/c1/855g8kxn12vcy4g6k97c_cc80000gn/T/ExercismTestFixture/ExercismTestFixture.xcodeproj
ERROR: Unexpected action: test

% objc HelloWorld -h
Usage: objc [options] FILE_PREFIX

    The `objc` binary requires that you provide a file prefix so that it is
    able to find the: header file; source file; and test file.

    $ objc Anagram

    This will find the following: Anagram.h; Anagram.m; and AnagramTest.m

Specific options:
    -x, --xcodebuild                 Use xcodebuild
    -h, --help                       Show this message
        --version                    Show version

% objc HelloWorld -x
▸ Building ExercismTestFixture/IgnoreThisTarget [Debug]
▸ Check Dependencies
▸ Processing IgnoreThisTarget-Info.plist
▸ Touching IgnoreThisTarget.app
▸ Building ExercismTestFixture/test-suite [Debug]
▸ Check Dependencies
▸ Test Succeeded
All tests
Test Suite test-suite.xctest started
HelloWorldTest
    ✓ testNoName (0.086 seconds)
    ✓ testOtherSampleName (0.001 seconds)
    ✓ testSampleName (0.000 seconds)


         Executed 3 tests, with 0 failures (0 unexpected) in 0.087 (0.088) seconds

[burtlo]

Hey thanks for keeping this up and running. Merged and pushed 0.0.4