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

[pnorman]

Whoops - this only applies to postgis 2.0 or later. The matrix of install methods is more complicated when you include 1.5

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/osm2pgsql/pull/24#issuecomment-17795339
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