Only injects code into the first occurrence of <head> tags

This partially addresses issue #60

I think the idea was that https://github.com/johnbintz/rack-livereload/blob/master/lib/rack/livereload/body_processor.rb#L73 would prevent this issue from occurring.
However RACK can pass the entire document as a single line. So when entering this section of code the @livereload_added would still be false and then it would proceed to replace all occurrences of the text <head> with the injected code.

This fix simply makes sure we replace only the first occurrence of <head> as I believe was intended by the check found on line 73 of the body_processor.rb.