While I can't comment on Travis CI, Gitlab from what I understand, used to hire a 3rd party company that would perform research (based on cost of living, taxes etc.) to gauge your salary (across different regions).

That said, what's stopping remote engineers starting to work in expensive regions (say Vancover/Toronto) and then moving to cheaper regions (Saskatchewan/Manitoba/Alberta) for example - would salaries be adjusted every time someone moves around?

In my opinion, one of the biggest benefits of working remotely would be the ability to travel the world!