Only match numbers in mkvtoolnix version check

Some versions of mkvmerge return "v7.0.0" as the version number. With the old sed
matcher, the "v" in the expression was not filtered out and therefore the
wrong audio and video track prefixes would be used. Now only alpha–numeric
characters are returned.
Merge pull request #101 from treppo/master

Only match numbers in mkvtoolnix version check