Fix compile error with es6 target

ndarray/ndarray-test.ts

@@ -1,6 +1,6 @@
 /// <reference path="ndarray.d.ts" />
 
-import ndarray = require('ndarray');
+import * as ndarray from 'ndarray';
 
 const data = new Int32Array(2 * 2 * 2 + 10);
 const a = ndarray(data, [2, 2, 2], [1, 2, 4], 5);

ndarray/ndarray.d.ts

@@ -31,5 +31,6 @@ declare module 'ndarray' {
 
   function ndarray(data: Data, shape?: number[], stride?: number[], offset?: number): ndarray;
 
+  namespace ndarray { }
   export = ndarray;
 }