convert-ly and multiple versions of Python

The LilyPond convert-ly script lives here under a standard OS X install of LilyPond:

   /Applications/LilyPond.app/Contents/Resources/bin/convert-ly

The script is written in Python but starts with the following directive to the shell:

   #!/usr/bin/python

If you have multiple versions of Python installed on your system, convert-ly may find an old version of Python and be unable to start. Fix this by changing the first line of convert-ly to this:

   #!/usr/bin/env python