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 make_self_deleting_tempfile() {
   case $(uname -s) in
     Darwin )
       : ${TMPDIR:=/tmp} ;
-      name=$(mktemp -t _stacklib_ )
+      name=$(mktemp -t _stacklib_.XXXXXXX )
       ;;
     Linux )
       name=$(mktemp)