workaround for recent Zygote break

FluxML/Zygote.jl#924 (comment)

Author: marius311

Project.toml

@@ -83,5 +83,5 @@ StaticArrays = "0.12.1, 1.0"
 StatsBase = "0.32, 0.33"
 TimerOutputs = "0.5"
 UnPack = "1"
-Zygote = "0.4.14, 0.5, 0.6"
+Zygote = "0.4.14, 0.5, 0.6 - 0.6.4"
 julia = "1.5"