define JSON.lower(CatValue) if JSON is available

fixes JuliaData#95

REQUIRE

@@ -1,3 +1,4 @@
 julia 0.6
 Nulls 0.1.2
 Reexport
+Requires

src/CategoricalArrays.jl

@@ -14,6 +14,7 @@ module CategoricalArrays
     using Compat
     using Reexport
     @reexport using Nulls
+    using Requires
 
     include("typedefs.jl")
 

src/value.jl

@@ -168,3 +168,10 @@ 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