Add compatibility with mkvtoolnix v6.7.0

As has been noted in issue #93, the current version of mkvdts2ac3 is incompatible with mkvtoolnix v6.7.0. This is due to the omission of the "A_" and "V_" prefixes from the identifiers of audio and video tracks.

I've attached a simple patch that checks the version of the installed mkvtoolnix and adjusts the search expressions correspondingly.