Fix plural calculation for Icelandic.

src/I18n/PluralRules.php

@@ -66,7 +66,7 @@ class PluralRules {
 		'hr' => 3,
 		'hu' => 1,
 		'id' => 0,
-		'is' => 1,
+		'is' => 15,
 		'it' => 1,
 		'ja' => 0,
 		'jv' => 0,
@@ -191,6 +191,8 @@ class PluralRules {
 				return $n == 1 ? 0 :
 					($n == 2 ? 1 :
 					($n != 8 && $n != 11 ? 2 : 3));
+			case 15:
+				return ($n % 10 != 1 || $n % 100 == 11) ? 1 : 0;
 		}
 	}
 

tests/TestCase/I18n/PluralRulesTest.php

@@ -61,6 +61,11 @@ class PluralRulesTest extends TestCase {
 			['ga', 2, 1],
 			['ga', 7, 3],
 			['ga', 11, 4],
+			['is', 1, 0],
+			['is', 2, 1],
+			['is', 3, 1],
+			['is', 11, 1],
+			['is', 21, 0],
 			['lt', 0, 2],
 			['lt', 1, 0],
 			['lt', 2, 1],