TagInfo's order field should be a string. Fixes #4.

src/tag.rs

@@ -19,5 +19,5 @@ pub struct TagEventContent {
 pub struct TagInfo {
     /// Value to use for lexicographically ordering rooms with this tag.
     #[serde(skip_serializing_if="Option::is_none")]
-    pub order: Option<u64>,
+    pub order: Option<String>,
 }