[MacPorts] #43222: mkvtoolnix: upgrade to version 6.8.0

[MacPorts]

#43222: mkvtoolnix: upgrade to version 6.8.0
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  Reporter:  dgonyier@…  |      Owner:  macports-tickets@…
      Type:  update      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:
      Port:  mkvtoolnix  |
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Comment (by mojca@…):

 Unrelated, but probably worth fixing – the build fails for me on a
 slightly "cleaner" machine:
 {{{
 --->  Building mkvtoolnix
 rake aborted!
 Ruby 1.9.x or newer is required for building
 }}}
 I had to fix `rake.d/vendor/drake-0.9.2.0.3.1/bin/drake` to launch the
 right ruby version.

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Ticket URL: <https://trac.macports.org/ticket/43222#comment:3>
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