Change the input type of defineMessages to T

This allows for full intellisense on the return value, so `messages.foo` will work.

react-intl/react-intl.d.ts

@@ -24,7 +24,7 @@ declare module ReactIntl {
         [key: string]: FormattedMessage.MessageDescriptor
     }
 
-    function defineMessages<T extends Messages>(messages: Messages): T;
+    function defineMessages<T extends Messages>(messages: T): T;
 
     interface IntlShape extends React.Requireable<any> {
     }
@@ -236,4 +236,4 @@ declare module "react-intl" {
 declare module "react-intl/lib/locale-data/en" {
     var data: ReactIntl.LocaleData;
     export = data;
-}
+}