[MacPorts] #55461: postgis2: fix license

[MacPorts]

#55461: postgis2: fix license
------------------------+----------------------
 Reporter:  ryandesign  |      Owner:  Veence
     Type:  defect      |     Status:  new
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:  postgis2
------------------------+----------------------
 The LICENSE.TXT file in postgis 2.4.0 says:

 {{{
 In general PostGIS may be distributed and/or modified under the conditions
 of the GNU General Public License, either version 2 or (at your option)
 any
 later version. Please refer to the COPYING file for details.
 }}}

 As such, please change the `license` line of the port from `GPL` to
 `GPL-2+`.

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Ticket URL: <https://trac.macports.org/ticket/55461>
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