I followed the instructions in webtest/README.md, but when I execute ./node_modules/.bin/wdio wdio.conf.js, I get the following error repeated many times:

ERROR: unknown error: no chrome binary at /usr/bin/google-chrome
  (Driver info: chromedriver=2.40.565386 (45a059dc425e08165f9a10324bd1380cc13ca363),platform=Mac OS X 10.13.3 x86_64)
chrome
    at new RuntimeError (/Users/aretter/code/tei-publisher-app/webtest/node_modules/webdriverio/build/lib/utils/ErrorHandler.js:143:12)
    at Request._callback (/Users/aretter/code/tei-publisher-app/webtest/node_modules/webdriverio/build/lib/utils/RequestHandler.js:316:39)
    at Request.self.callback (/Users/aretter/code/tei-publisher-app/webtest/node_modules/request/request.js:185:22)
    at Request.emit (events.js:127:13)
    at Request.emit (domain.js:421:20)
    at Request.<anonymous> (/Users/aretter/code/tei-publisher-app/webtest/node_modules/request/request.js:1157:10)
    at Request.emit (events.js:127:13)
    at Request.emit (domain.js:421:20)
    at IncomingMessage.<anonymous> (/Users/aretter/code/tei-publisher-app/webtest/node_modules/request/request.js:1079:12)
    at Object.onceWrapper (events.js:219:13)

As I am on Mac OS, I would not expect there to be a /usr/bin/google-chrome.

What am I doing wrong?