[Tile-serving] [osm2pgsql] Update CREATE EXTENSION information (#24)

[pnorman]

Perhaps we should switch the default descriptions in the readme from 8.4 to 9.1?
You can merge this Pull Request by running:

  git pull https://github.com/pnorman/osm2pgsql patch-1

Or you can view, comment on it, or merge it online at:

  https://github.com/openstreetmap/osm2pgsql/pull/24

-- Commit Summary --

  * Update CREATE EXTENSION information

-- File Changes --

    M README (5)

-- Patch Links --

https://github.com/openstreetmap/osm2pgsql/pull/24.patch
https://github.com/openstreetmap/osm2pgsql/pull/24.diff

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