Update validation.php

Language validation addition for https://github.com/laravel/framework/pull/4599

app/lang/en/validation.php

@@ -27,6 +27,7 @@ return array(
 		"string"  => "The :attribute must be between :min and :max characters.",
 		"array"   => "The :attribute must have between :min and :max items.",
 	),
+	"boolean"              => "The :attribute field must be either true or false",
 	"confirmed"            => "The :attribute confirmation does not match.",
 	"date"                 => "The :attribute is not a valid date.",
 	"date_format"          => "The :attribute does not match the format :format.",