Summary:
- making website deployment and desktop app builds two different travis jobs, so if one fails, the other is still working
- running website builds on linux, because, we don't need macOS for this
- making dependency installs serial instead of parallel to avoid problems of multiple yarn instances running at the same time
- removing duplicate postinstall script which breaks for electron 3 builds
Reviewed By: passy
Differential Revision: D9148342
fbshipit-source-id: 22f3a0fe59aff066c04fb964621604efc46aab8a
696c73ebc1