Fix issue on OS X when creating a temporary file

This prevents the "mktemp: too few X's in template
_stacklib_" error message from appearing when
adding another admin to blackbox and allows the command to successfully
execute.

    Related:
    rtomayko/shocco#5
    http://unix.stackexchange.com/q/87638/63283

bin/_stack_lib.sh

@@ -55,7 +55,7 @@ function create_self_deleting_tempfile() {
   case $(uname -s) in
     Darwin )
       : ${TMPDIR:=/tmp} ;
-      filename=$(mktemp -t _stacklib_ )
+      filename=$(mktemp -t _stacklib_.XXXXXXXX )
       ;;
     Linux )
       filename=$(mktemp)