issue with eiconv

[jschoch]

not sure if this is somehow related to hex

14:31:43.337 [error] The on_load function for module eiconv returned {{:badmatch,
  {:error,
   {:load_failed,
    'Failed to load NIF library: \'/home/bitnami/tmp/mail-to-json/_build/dev/lib/eiconv/priv/eiconv_nif.so: undefined symbol: libiconv\''}}},
 [{:eiconv, :init, 0, [file: 'src/eiconv.erl', line: 18]},
  {:code_server, :"-handle_on_load/4-fun-0-", 1,
   [file: 'code_server.erl', line: 1659]}]}