Pin protobuf to version 2.18
This temporarily pins the version of protobuf because 2.19 introduces
some deprecation warnings that fail our build (because warnings are
fatal).
@@ -49,7 +49,7 @@ mio-extras = "2"
oauth2 = { version = "3.0", optional = true }
openssl = "0.10"
percent-encoding = { version = "2.0", optional = true }
-protobuf = "2"
+protobuf = "~2.18"
rand = "0.7"
reqwest = { version = "0.10", optional = true, features = ["blocking", "json"] }
serde = "1.0"
