Update sentry-actix to work with latest actix-web

[gabelerner]

It's currently pinned to 0.7
https://github.com/getsentry/sentry-rust/blob/master/integrations/sentry-actix/Cargo.toml#L30

actix-web is up to 2.0.0

Because the actix_web::middleware::Middleware trait is gone, I can't even compose the old version into the new one. Example that fails:

[onelson]

I suspect nothing has changed to resolve the blockers identified in #143. Most of the difference between actix-web 1.0 and 2.0 came in the transition to async/await.

[gabelerner]

you're right, i will close and wait for that one!