[wsg] replace .for with .flatmap

The method .for is deprecated and will be removed in Rakudo 2015.09.  Its
replacement is .flatmap as corrected here.

Author: paultcochrane

categories/wsg/advanced-2008/event010-dwhipp.p6

@@ -150,7 +150,7 @@ sub MAIN(Bool :$computer-player = False) {
 
     my @suites = < spades clubs diamonds hearts >;
 
-    my @deck = ( @values X @suites ).for: {
+    my @deck = ( @values X @suites ).flatmap: {
         my ($name, $value) = $^a.kv; $name ~= " of $^b"; $name => $value
     };