Summary:
build-client, aka packaging, aka bundling is now separate from uploading.
This is both to compartmentalize our tasks a bit more and so we can add a
non-grunt windows task inbetween packaging and uploading.
No more heavily-overloaded PUBLISH_BUILD flag.
Added SIGN_BUILD flag instead.
No more TRAVIS and TRAVIS_PULL_REQUEST flag.
Test Plan: Manual
Reviewers: halla, juan, spang
Reviewed By: juan, spang
Differential Revision: https://phab.nylas.com/D4208
