self.xcode_version should use xcrun

see: man xcrun /DEVELOPER_DIR - we need to respect this env var
should ask instruments for path to Automation template

 As of Xcode 4.6.3 we can do this:

 instruments -s templates and search for Automation.tracetemplate
  +1 drop support for Xcode < 5.0
  -1 instruments can be _slow_
  +1 for simplicity
  -1 for Xcode < 5.1 complexity
  +1 for respecting the DEVELOPER_DIR env. var.
  +1 for the possibility of supporting the next version of Xcode
  --
  +2
should mention run_loop requires Xcode >= 5.0
gem should be versioned at 1.0.0.pre1

http://semver.org/
* If your software is being used in production, it should probably
  already be 1.0.0.
* If you have a stable API on which users have come to depend, you
  should be 1.0.0.
* If you're worrying a lot about backwards compatibility, you should
  probably already be 1.0.0.