Update BN import (#2431)

As mentioned in MetaMask/core#428 the `BN` export was removed in MetaMask/core#398. This is the only instance where it's being imported this way. We should update this now as the `BN` export will be removed from `controllers` in a future version.

app/components/Views/ApproveView/Approve/index.test.js

@@ -2,7 +2,7 @@ import React from 'react';
 import { shallow } from 'enzyme';
 import Approve from './';
 import configureMockStore from 'redux-mock-store';
-import { BN } from '@metamask/controllers';
+import { BN } from 'ethereumjs-util';
 
 const mockStore = configureMockStore();