Add specific iOS simulator destination
Add specific iOS simulator destination

Specifying the destionation works around the following issue with xctool. Can also hard-code the architecture, but this seems more future-proof.

```
Failed to query the list of test cases in the test bundle: 2015-08-24 10:57:11.740 sim[50821:2746840] /Applications/Xcode6.4.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/sim: No simulator devices appear to be running.  Setting data directories to /var/empty.
2015-08-24 10:57:11.741 sim[50821:2746840] DYLD_INSERT_LIBRARIES contains possible bad values.  Caller beware: /usr/local/Cellar/xctool/0.2.4/libexec/lib/otest-query-lib-ios.dylib
dlopen(/Volumes/DerivedData/ObjectiveGitFramework-drtqacfhlqybdcdpqayztrrwneun/Build/Products/Debug-iphonesimulator/ObjectiveGit-iOSTests.xctest/ObjectiveGit-iOSTests, 1): Library not loaded: @rpath/ObjectiveGit.framework/ObjectiveGit
  Referenced from: /Volumes/DerivedData/ObjectiveGitFramework-drtqacfhlqybdcdpqayztrrwneun/Build/Products/Debug-iphonesimulator/ObjectiveGit-iOSTests.xctest/ObjectiveGit-iOSTests
  Reason: no suitable image found.  Did find:
	/Volumes/DerivedData/ObjectiveGitFramework-drtqacfhlqybdcdpqayztrrwneun/Build/Products/Debug-iphonesimulator/ObjectiveGit.framework/ObjectiveGit: mach-o, but wrong architecture
```

Indeed, the "wrong architecture" message is correct as the framework was built for a different architecture than the test binary:

```
lipo -info ObjectiveGit.framework/ObjectiveGit ObjectiveGit-iOSTests.xctest/ObjectiveGit-iOSTests                                                                                                                                        2015-08-24 11:03
Non-fat file: ObjectiveGit.framework/ObjectiveGit is architecture: i386
Non-fat file: ObjectiveGit-iOSTests.xctest/ObjectiveGit-iOSTests is architecture: x86_64
```

tiennou/objective-git,nerdishbynature/objective-git,phatblat/objective-git,blackpixel/objective-git,slavikus/objective-git,libgit2/objective-git,dleehr/objective-git,dleehr/objective-git,alehed/objective-git,phatblat/objective-git,dleehr/objective-git,slavikus/objective-git,tiennou/objective-git,nerdishbynature/objective-git,blackpixel/objective-git,0x4a616e/objective-git,dleehr/objective-git,phatblat/objective-git,blackpixel/objective-git,alehed/objective-git,libgit2/objective-git,slavikus/objective-git,tiennou/objective-git,libgit2/objective-git,libgit2/objective-git,alehed/objective-git,0x4a616e/objective-git,nerdishbynature/objective-git,blackpixel/objective-git,0x4a616e/objective-git