NodeJS: Mocha+Selenium tests with promises fail on Travis (succeed on local)

I'm doing automated Mocha testing with Selenium WebDriver on Travis. If I just run the test synchronously and call done() at the end, the tests pass beautifully. As soon as I introduce async behavior (ie. .then(function(){... done()}), Travis tests fail, although locally they still run as they're supposed to.

For example:

describe('Map load test', function() {
    let driver = new webdriver.Builder()
        .forBrowser('firefox')
        .build();
    this.timeout(60000);

    beforeEach(function(done) {
        console.log('beforeEach');
        driver.get(index).then(function() {
            console.log('Loaded index');
            done();
        });
    });

    after(function() {
        console.log('after');
        driver.quit();
    });

    it('Should load the choropleth demo without issues', function(done) {
        console.log('Starting Choropleth test');
        driver.getCurrentUrl().then(function(url) {
            assert.equal(url, index, 'Initial url did not match');
            console.log('loaded' + url);
            driver.findElement(By.css('#demo0')).click();
            driver.findElement(By.css('.loadDemoButton')).click();
            driver.sleep(3000);
            console.log('slept for 3000');
            driver.getCurrentUrl().then(function(url) {
                assert.equal(url, index + '#edit', 'File loaded url did not match');
                done();
            });
        });

    });

Results in (Note - doesn't even go into the actual test but fails on beforeEach)

Map load test

beforeEach

    1) "before each" hook for "Should load the choropleth demo without issues"

after

  0 passing (1m)

  1 failing

  1) Map load test "before each" hook for "Should load the choropleth demo without issues":

     Error: Timeout of 60000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

But doing

beforeEach(function(done) {
        console.log('beforeEach');
        driver.get(index).then(done());
    });

continues on to the next test.

Travis.yml:

language: node_js
addons:
  firefox: "latest"
node_js:
 - "6.9.0"
 - "7.6.0"
before_install:
  - wget https://github.com/mozilla/geckodriver/releases/download/v0.13.0/geckodriver-v0.13.0-linux64.tar.gz
  - mkdir geckodriver
  - tar -xzf geckodriver-v0.13.0-linux64.tar.gz -C geckodriver
  - export PATH=$PATH:$PWD/geckodriver

The test file

Failing Travis build logs

Thanks for any suggestions. Debugging Travis failures is such a PITA...

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