Build: Adjust build chain for Travis/SauceLabs

- Minor description updates and better differentiate internal tasks
- Make the SauceLabs an self contained external task
- Clean up assemble task and add "tests" target for SauceLabs/Mocha runner

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@patheard, @LaurentGoderre: Not sure if this will fix all the Sauce issues we're hitting, but I'll let you guys comment before merging