more JSON compat

Author: pfitzseb

Project.toml

@@ -14,7 +14,7 @@ WebIO = "0f1e0344-ec1d-5b48-a673-e5cf874b6c29"
 [compat]
 julia = "0.7, 1"
 JSExpr = "1"
-JSON = "0.20"
+JSON = "0.18, 0.19, 0.20, 0.21"
 Observables = "0.2"
 Tables = "1"
 WebIO = "0.8"