Update Project.toml (#456)

Author: pfitzseb

Project.toml

@@ -36,7 +36,7 @@ Widgets = "0.6.2"
 julia = "0.7, 1"
 JSExpr = "0.5"
 JSON = "0.18, 0.19, 0.20, 0.21"
-Observables = "0.2.3, 0.3"
+Observables = "0.2.3, 0.3, 0.4"
 FunctionalCollections = "0.5.0"
 AssetRegistry = "0.1.0"
 WebSockets = "1.5.0"