Update 'handlebars' to 0.28.

Author: Sean Stangl

contrib/Cargo.toml

@@ -33,7 +33,7 @@ serde_json = { version = "1.0", optional = true }
 rmp-serde = { version = "^0.13", optional = true }
 
 # Templating dependencies only.
-handlebars = { version = "^0.27", optional = true }
+handlebars = { version = "^0.28", optional = true }
 glob = { version = "^0.2", optional = true }
 tera = { version = "^0.10", optional = true }