juniper_warp semver breaking release

[Visic]

The juniper_warp release 0.6.5 includes the update of tokio from 0.2 to version 1 which was introduced in the juniper_warp 0.7.0 release. The end result being that anyone using a 0.6 version of juniper_warp along with a version of tokio compatible with that release (e.g. tokio 0.2), will not be able to build due to mismatched tokio requirements.