Increased testem's `browser_start_timeout` to 120s

no issue

- we're seeing random failures in CI with the error "Browser failed to connect within 30s. testem.js not loaded?"
- bumped the timeout to 120s to determine if it's due to occasional CI-related slowness or something else

testem.js

@@ -5,6 +5,7 @@ let launch_in_ci = [process.env.BROWSER || 'Chrome'];
 
 module.exports = {
     framework: 'mocha',
+    browser_start_timeout: 120,
     browser_disconnect_timeout: 60,
     test_page: 'tests/index.html?hidepassed',
     disable_watching: true,