Saucelabs config: set screen resolution and OS

Summary:
* set screen resolution to 1280x1024
  * because the current responsive layout moves the sign in button to the left, mobile-styled collapsible menu
    * so the current e2e specs are failing
      * in the long term, we could also write specific specs for specific screen resolutions/platforms/browsers
* set platform to Windows 8
  * Linux on Saucelabs only support screen resolution of 1024x768
  * using Windows might also be a good choice for a better coverage of multiple platforms
    * because most developers probably use Mac or Linux anyway

Test Plan: * check if TravisCI build is now completely green

Reviewers: shadowhand

Reviewed By: shadowhand

Subscribers: aMoniker, seth, rjmackay

Differential Revision: https://phabricator.ushahidi.com/D498