[install] OS X section update

CocoCaml was last updated in 2004, so it only has binaries for PPC. Maybe it can be resurrected and recompiled for x86, but right now it doesn't work.

xCode plugin was last updated in 2008 and only works with XCode 3.x.

INRIA binary package for 4.0.1 says it wants OS X 1.7.x and XCode tools 4.3.3. I have XCode 4.2 installed and it seems to work, however, I relay the official requirements here.