[MacPorts] #49343: slime: update to 2.15

[MacPorts]

#49343: slime: update to 2.15
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 Reporter:  ryandesign@…  |      Owner:  easieste@…
     Type:  update        |     Status:  new
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.3.99
 Keywords:  haspatch      |       Port:  slime
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 This patch updates slime to 2.15. I am not committing it because I have
 not tested it, as the comment at the top of the portfile says to do,
 because I don't know what the testing procedure is.

 The port gave no explanation for why it was fetching from a git
 repository, so I changed it to use a github tarball instead, and it built
 ok for me.

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Ticket URL: <https://trac.macports.org/ticket/49343>
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