Improve apns_utils:sing/1 in order to use os:cmd/1 instead of ktn_os:command/1

[cabol]

NOTE: This issue is related to inaka/erlang-katana#149

As @getong points out in #157, there is a race condition that was fixed in os:cmd here.

We still need ktn_os:command because we want to find out the exit status code of the command we're running. The idea is to replace ktn_os:command/1 by os:cmd/1, but wrapping os:cmd/1 function in another one adding the necessary logic to validate if the command was executed successfully or not.

Possible solution:

-spec cmd(string()) -> {0 | 1, string()}.
cmd(Cmd) ->
  NewCmd = "
    R=$(" ++ Cmd ++ ")\n
    if [ $? -eq 0 ];then\n
      echo \"0::$R\"\n
    fi",
  case string:tokens(os:cmd(NewCmd), "::") of
    ["0", Result] -> {0, Result};
    Error         -> {1, Error}
  end.