Speeded up PC centaurs to (almost) match monster centaur movement rate,

per 1693565. This could have balance issues; speak up if you think it's bad.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@1216 c06c8d41-db1a-0410-9941-cceddc491573

Created by  haranp  on April 4, 2007
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