[bash/custom-prompt] decrement JOBS_NUM if autojump is installed.

It not because of "current job" as I think before, it because of
"autojump" add next string to "PROMPT_COMMAND":

azat:~$ echo $AUTOJUMP { [[ "$AUTOJUMP_HOME" == "$HOME" ]] && (autojump -a "$(pwd ${_PWD_ARGS})"&)>>/dev/null 2>>"${AUTOJUMP_DATA_DIR}/.autojump_errors";} 2>>/dev/null