[MacPorts] #56110: Travis: skip building ports from the obsolete PortGroup

[MacPorts]

#56110: Travis: skip building ports from the obsolete PortGroup
----------------------------+------------------
  Reporter:  mojca          |      Owner:  l2dy
      Type:  defect         |     Status:  new
  Priority:  Low            |  Milestone:
 Component:  buildbot/mpbb  |    Version:
Resolution:                 |   Keywords:
      Port:                 |
----------------------------+------------------

Comment (by raimue):

 In [changeset:"868f68814515c2af4dc4c4921b489b7ded410e99/mpbb"
 868f68814515c2af4dc4c4921b489b7ded410e99/mpbb] (master):
 {{{
 #!ConfigurableCommitTicketReference repository="mpbb"
 revision="868f68814515c2af4dc4c4921b489b7ded410e99"
 mpbb-list-subports: Allow empty --archive-site

 Passing an empty string to --archive-site disables the checks on the
 remote mirror.

 See: https://trac.macports.org/ticket/56110
 }}}

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Ticket URL: <https://trac.macports.org/ticket/56110#comment:4>
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