Move to BrowserStack

Created by: Johann-S

Related to our internal discussion about moving away from Sauce Labs to BrowserStack

• Register us on BrowserStack here: https://www.browserstack.com/open-source
• Add 2 new env vars in Travis BROWSER_STACK_USERNAME and BROWSER_STACK_ACCESS_KEY
• To fill those vars, you'll found that here https://www.browserstack.com/accounts/automate
• Add BrowserStack badge
• Fix mobile testing
  • Android
  • iOS
• When we are all done, remove the old Sauce Labs Travis variables