Support the xcode_sdk setting

By adding this to your .travis.yml:

    xcode_sdk:
      - iphonesimulator6.0
      - iphonesimulator6.1

The tests will be run using the iOS 6.0 and 6.1 SDKs.
Allow Xcode schemes to be specified in the Travis config

Normally the build script tries to be smart about what scheme to run,
but sometimes this must be overridden. A scheme can now be specified
like this:

    xcode_scheme: MyProjectTests

xcode_scheme is also a matrix key, so you could do this to test multiple
schemes:

    xcode_scheme:
      - MyProjectUnitTests
      - MyProjectAppTests