Mock webpack in index to prevent require error

Author: Frank Schmid

index.test.js

@@ -29,6 +29,7 @@ describe('ServerlessWebpack', () => {
 
     mockery.enable({ useCleanCache: true, warnOnUnregistered: false });
     mockery.registerMock('ts-node/register', {});
+    mockery.registerMock('webpack', {});
 
     ServerlessWebpack = require('./index');
     sandbox.spy(Module, '_load');