Use the new "# sourceMappingURL=..." comment format when referencing source maps

According to https://docs.google.com/a/chromium.org/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k,
the preferred way is to use "#", since "@" conflicts with IE's conditional comments.