(bug 56859) Update $wgCategoryCollation on iswiki

This patch sets $wgCategoryCollation to 'uca-is' on
iswiki (the Icelandic Wikipedia) per community request;
see bug for URL with on-wiki proposal for this change.

Bug: 56859
Change-Id: Ic36cd656233dfc959c92a33bade794cc5c1e1bd2