Allow JSON@0.21

Author: odow

Project.toml

@@ -23,7 +23,7 @@ BenchmarkTools = "1"
 CodecBzip2 = "0.6, 0.7, 0.8"
 CodecZlib = "0.6, 0.7"
 ForwardDiff = "0.10, 1"
-JSON = "1"
+JSON = "0.21, 1"
 JSONSchema = "1"
 LinearAlgebra = "1"
 MutableArithmetics = "1"