fix(growth): use Application.app_dir to write/read priv dir

lib/growth/indicators/download.ex

@@ -321,6 +321,8 @@ defmodule Growth.Indicators.Download do
   end
 
   def save(data, measurement) do
-    File.write("priv/growth/indicators/#{measurement}.csv", data)
+    :wabanex
+    |> Application.app_dir(["priv", "growth", "indicators", "#{measurement}.csv"])
+    |> File.write(data)
   end
 end