Use /usr/bin/env to support non standard ruby locations

Please consider following patch as suggested in the comment at web page http://www.clearchain.com/blog/posts/splitting-a-patch "As an aside, consider changing the shebang to use ‘env’:
#!/usr/bin/env ruby so that people like me who have ruby in other locations don’t have to modify it."

Signed-off-by: Jari Aalto <jari.aalto@cante.net>