Why isn't Jest mocking my dependencies?

I can't seem to get Jest to mock my dependencies.

package.json:

{
  ...
  "scripts": {
    "start": "webpack && concurrently --kill-others 'webpack --watch' 'nodemon build/server.js'",
    "test": "jest"
  },
  "author": "Jonathan Conway",
  "license": "ISC",
  "devDependencies": {
    "babel-jest": "^15.0.0",
    "babel-polyfill": "^6.13.0",
    "jest": "^15.1.1",
    "jest-cli": "^15.1.1"
  },
  ...
  "jest": {
    "scriptPreprocessor": "jest-script-preprocessor.js",
    "testRegex": "(/__tests__/.*|\\.(test|spec))\\.(es6|js|json)$"
  }
}

src/app.js:

import express from 'express';

let app = express();

app.set('view engine', 'ejs');
app.set('views', '');

app.use('/build/public', express.static('build/public'));

export default app;

src/components/home/home.routes.js:

import app from '../../app';

export default class HomeRoutes {
  constructor () {
    app.get('/', (req, res) => {
        res.render('src/components/home/home');
    });
  }
}

src/components/home/home.test.js:

jest.mock('../../app');
const app = require('../../app');
const HomeRoutes = require('./home.routes');

it('works', () => {
  app.get = jest.genMockFunction();
  var homeRoutesInstance = new HomeRoutes.default();
  console.log(app.get.mock.calls.length); // outputs 0
});

When I run npm test on this, I gete 0 calls on the app.get function, despite the HomeRoutes constructor calling app.get.

Am I missing something here? Perhaps some extra configuration of Jest is required?

Thanks!