define JSON.lower(CatValue) if JSON is available

fixes #95

REQUIRE

@@ -2,3 +2,4 @@ julia 0.6
 Missings
 Reexport
 Compat 0.39.0
+Requires

src/CategoricalArrays.jl

@@ -18,6 +18,7 @@ module CategoricalArrays
     if VERSION >= v"0.7.0-DEV.3052"
         using Printf
     end
+    using Requires
 
     include("typedefs.jl")
 

src/value.jl

@@ -160,3 +160,10 @@ Base.match(r::Regex, s::CategoricalString,
 Base.matchall(r::Regex, s::CategoricalString, overlap::Bool=false) =
     matchall(r, get(s), overlap)
 Base.collect(x::CategoricalString) = collect(get(x))
+
+@require JSON begin
+
+# Serialize CatValue as the value it refers to (fixes #95)
+JSON.lower(x::CatValue) = get(x)
+
+end