Update deps to fix automated Firefox testing

[jeffposnick]

The Travis CI build is currently failing with

WebDriverError: Unable to parse new session response: {"value": {"sessionId":"6ddc6985-10fb-4336-bef1-5e9294314a77","value":{"acceptInsecureCerts":false,"browserName":"firefox","browserVersion":"52.0.2","moz:accessibilityChecks":false,"moz:processID":5940,"moz:profile":"/tmp/rust_mozprofile.ECkbedt6jzEs","pageLoadStrategy":"normal","platformName":"linux","platformVersion":"4.4.0-51-generic","rotatable":false,"specificationLevel":0,"timeouts":{"implicit":0,"page load":300000,"script":30000}}}}

This should (hopefully!) fix it, based on what @gauntface did in https://github.com/GoogleChrome/sw-testing-helpers/commit/8363744ff63f754978648e04c48de0821b53a535