use ES6 import instead of require to pull in 'sum' to sum-test

Author: bdsabian

src/__tests__/sum-test.js

@@ -1,6 +1,6 @@
 jest.dontMock('../sum'); // or jest.autoMockOff();
 
-var sum = require('../sum')['default'];
+import sum from '../sum';
 
 describe('sum', function () {
     it('adds 1 + 2 to equal 3', function () {