Revert "Update ring to work with latest nightly rust"

This reverts commit c415ef2.

This was fixed in nightly:
rust-lang/rust#47298

Author: musoke

Cargo.toml

@@ -13,11 +13,11 @@ version = "0.4.0"
 [dependencies]
 base64 = "0.5.0"
 lazy_static = "0.2.8"
-ring = "0.13.0-alpha"
+ring = "0.9.4"
 ruma-identifiers = "0.11.0"
 serde = "1.0"
 serde_json = "1.0"
-untrusted = "0.6"
+untrusted = "0.5"
 url = "1.4"
 
 [dev-dependencies]