fix(power-assert): use CommonJS style default export statement

power-assert/power-assert-tests.ts

@@ -1,6 +1,6 @@
 /// <reference path="./power-assert.d.ts" />
 
-import assert from "power-assert";
+import assert = require("power-assert");
 
 assert(1 + 1 - 2 === 0, "The universe isn't how it should.");
 

power-assert/power-assert.d.ts

@@ -66,5 +66,5 @@ declare namespace assert {
 }
 
 declare module "power-assert" {
-    export default assert;
+    export = assert;
 }