Change method names in Spring convenience class

Users will find it more friendly and easier to remember if all they need is a
static import and a method called "spring" (as opposed to having to remember 2
method names, one of which is "run" which could mean anything).

E.g.

ratpack {
  handlers {
    register spring(SampleSpringBootApp)

     handler("foo") { String msg ->
      render msg
    }
  }
}