Make Message::nonce a serde_json::Value

Nonces can actually be almost anything - including booleans - so just
use a Value to represent it since very few users will need it.

This fixes errors like:
```
WARN:serenity::internal::ws_impl: (╯°□°）╯︵ ┻━┻ Error decoding: {"t":"MESSAGE_CREATE","s":12187872,"op":0,"d":{"type":0,"tts":false,"timestamp":"2017-06-01T01:00:00.000000+00:00","pinned":false,"nonce":"","mentions":[{"username":"redacted","id":"redacted","discriminator":"redacted","avatar":"redacted"}],"mention_roles":[],"mention_everyone":false,"id":"redacted","embeds":[],"edited_timestamp":null,"content":"redacted","channel_id":"redacted","author":{"username":"redacted","id":"redacted","discriminator":"redacted","bot":true,"avatar":"redacted"},"attachments":[]}}
ERROR:serenity::client: Shard handler received err: Json(ErrorImpl { code: Message("Unknown i64 value: "), line: 0, column: 0 })
```

and:

```
WARN:serenity::internal::ws_impl: (╯°□°）╯︵ ┻━┻ Error decoding: {"t":"MESSAGE_CREATE","s":1001192,"op":0,"d":{"type":0,"tts":false,"timestamp":"2017-06-01T01:01:01.000000+00:00","pinned":false,"nonce":true,"mentions":[],"mention_roles":[],"mention_everyone":false,"id":"redacted","embeds":[],"edited_timestamp":null,"content":"bork","channel_id":"redacted","author":{"username":"redacted","id":"redacted","discriminator":"redacted","bot":true,"avatar":"redacted"},"attachments":[]}}
ERROR:serenity::client: Shard handler received err: Json(ErrorImpl { code: Message("invalid type: boolean `true`, expected identifier"), line: 0, column: 0 })
```

1 files changed, 0 insertions, 31 deletions

diff --git a/src/model/utils.rs b/src/model/utils.rs
index 6363fb6..f88ecf5 100644
--- a/src/model/utils.rs
+++ b/src/model/utils.rs
@@ -240,34 +240,3 @@ impl<'de> Visitor<'de> for U64Visitor {
         Ok(v)
     }
 }
-
-pub struct I64Visitor;
-
-impl<'de> Visitor<'de> for I64Visitor {
-    type Value = i64;
-
-    fn expecting(&self, formatter: &mut Formatter) -> FmtResult {
-        formatter.write_str("identifier")
-    }
-
-    fn visit_str<E: DeError>(self, v: &str) -> StdResult<Self::Value, E> {
-        match v.parse::<i64>() {
-            Ok(v) => Ok(v),
-            Err(_) => {
-                let mut s = String::new();
-                s.push_str("Unknown i64 value: ");
-                s.push_str(v);
-
-                Err(DeError::custom(s))
-            },
-        }
-    }
-
-    fn visit_i64<E: DeError>(self, v: i64) -> StdResult<Self::Value, E> {
-        Ok(v)
-    }
-
-    fn visit_u64<E: DeError>(self, v: u64) -> StdResult<Self::Value, E> {
-        Ok(v as i64)
-    }
-}