Update typings for identify() function (#102)

It looks like identify() function's callback is missing arguments definitions:

https://github.com/launchdarkly/js-client/blob/078467e47c37def50030d29176141b908a5539ca/src/index.js#L150
https://github.com/launchdarkly/js-client/blob/078467e47c37def50030d29176141b908a5539ca/src/utils.js#L54

Author: apucacao

src/index.d.ts

@@ -220,7 +220,7 @@ declare module 'ldclient-js' {
      * @param onDone
      *   A callback to invoke after the user is identified.
      */
-    identify: (user: LDUser, hash?: string, onDone?: () => void) => Promise<void>;
+    identify: (user: LDUser, hash?: string, onDone?: (err: Error | null, flags: LDFlagSet | null) => void) => Promise<void>;
 
     /**
      * Flushes pending events asynchronously.