Deprecate support for Travis CI

[bertdeblock]

Wondering if Travis CI is still (widely) used across the ecosystem and if we should drop support for it.
I can't find many hits on emberobserver.com.

Thoughts?

[acorncom]

I think dropping support for it would be wise, believe Heroku getting hacked had to do with security issues at Travis 3+ months ago

[chriskrycho]

Yeah, agreed. Sad story but at this point the rest of it only gets sadder as far as I can see.

[sandstrom]

We've never used it. My hunch is that a lot of projects are moving to Github Actions.

Deprecating support won't make it impossible to use Travis CI either, so people can still use it if they want to. I guess this is mostly about removing some default files related to Travis?

[wagenet]

@bertdeblock do you want to write this RFC?

[bertdeblock]

@wagenet This was briefly discussed during one of the previous CLI meetings and the outcome was that it's not a priority and that some people might still be using Travis CI happily (only speculation of course). But I might end up writing a small RFC nonetheless, I may have some time in the coming weeks.

[bertdeblock]

Closing in favor of #918.