Switch from travis to Github Actions and setup bors

[therealprof]

Signed-off-by: Daniel Egger daniel@eggers-club.de

[eldruin]

I think this should only build on stable but not attempt to run the tests since they need nightly.

[therealprof]

Yeah, need to make the tests conditional somehow. This is clearly less than ideal to have CI depend on a nightly version.

[therealprof]

@ryankurte @ithinuel Can I get an approval here?

[eldruin]

Yes. One thing that bugs me specially is that await! includes yield in its expansion and I am not sure what kind of error you would get if attempting to use it on Rust stable. Probably something pretty weird.
I will have a look at adding a build.rs to only include it on nightly.

[therealprof]

@eldruin You should be able to approve this now. Want to give it a try? ;)

[eldruin]

Sounds good to me!