fix: Temporarily use firefox for testing

- https://travis-ci.com/github/frappe/frappe/jobs/488819006 testing is failing due to some incompatibility

.travis.yml

@@ -7,7 +7,7 @@ addons:
     - test_site_producer
   mariadb: 10.3
   postgresql: 9.5
-  chrome: stable
+  firefox: latest
 
 services:
   - xvfb

frappe/commands/utils.py

@@ -578,7 +578,7 @@ def run_ui_tests(context, app, headless=False):
 		frappe.commands.popen("yarn add cypress@^6 cypress-file-upload@^5 --no-lockfile")
 
 	# run for headless mode
-	run_or_open = 'run --browser chrome --record --key 4a48f41c-11b3-425b-aa88-c58048fa69eb' if headless else 'open'
+	run_or_open = 'run --browser firefox --record --key 4a48f41c-11b3-425b-aa88-c58048fa69eb' if headless else 'open'
 	command = '{site_env} {password_env} {cypress} {run_or_open}'
 	formatted_command = command.format(site_env=site_env, password_env=password_env, cypress=cypress_path, run_or_open=run_or_open)