Use updated //# pragma, fixes #22

See spec at https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#heading=h.lmz475t4mvbx
and discussion at https://groups.google.com/forum/#!msg/mozilla.dev.js-sourcemap/4uo7Z5nTfUY/_YNQtSxdclkJ