Make tests pass on OS X 10.10.1 with Ruby 2.1.5:

- Fix "NameError: uninitialized constant Timeout" in "waits until
  time runs out before stating 'Time to Rotate'"
- Instead of relying on the first `ruby` in $PATH being the right
  one to invoke, call the full path of the interpreter running us
- Let the project be checked out anywhere by assuming our current
  working directory is the top-level project dir, and make that
  assumption explicit

I'm expecting to mob in the usual RubySteps environment in the next
day or two, so if for some reason these changes wind up breaking
the tests there, I'll be the first to feel the pain and fix it.