Fix protobuf dependency invalid semver for cargo 1.54

With this, rust-lang/cargo#9508, the dependency can no longer be used.
What was picked with 1.53 cargo was ~2.0, so use that instead.

Test:
verify dependency can now be used

Cargo.toml

@@ -17,7 +17,7 @@ failpoint = ["fail"]
 
 [dependencies]
 log = ">0.2"
-protobuf = "~2.0-2.2"
+protobuf = "~2.0"
 quick-error = "1.2.2"
 rand = "0.5.4"
 hashbrown = "0.1"