Redirect to shields.io instead of writing our own

shields.io has supported docs.rs for a few months now. We don't support
as many features as they do, and they support all features we do, so
just forward requests on to them.

Author: jyn514

Cargo.lock

@@ -33,7 +33,6 @@ r2d2_postgres = "0.18"
 # iron needs url@1, but it reexports it as iron::url, so we can start using
 # url@2 for other usecases
 url = { version = "2.1.1", features = ["serde"] }
-badge = { path = "crates/badge" }
 docsrs-metadata = { path = "crates/metadata" }
 anyhow = { version = "1.0.42", features = ["backtrace"]}
 backtrace = "0.3.61"