Fix: incorrect field names for Attachment

I think I got the names from the Go source code, but the API emits JSON
that has these names instead. The api/swagger guide even says as much.

This caused the super fun quirk that the upload actually succeedes, but
the program reports an error condition because of the deserialization
failure. Time to bump a minor revision!

src/structs/mod.rs

@@ -9,7 +9,7 @@ pub struct Attachment {
     name: String,
     size: i64,
     download_count: i64,
-    created: String, // TODO: Date-time struct
+    created_at: String, // TODO: Date-time struct
     uuid: String,
-    download_url: String,
+    browser_download_url: String,
 }