more simple mail.sh fix

Reverted the original patch and changed it to the obvious simple solution provided by @Neilpang.

notify/mail.sh

@@ -78,13 +78,8 @@ mail_send() {
 
 _mail_bin() {
   _MAIL_BIN=""
-  _MAIL_BINS="sendmail ssmtp mutt mail msmtp"
 
-  if [ -n "$MAIL_BIN" ]; then
-    _MAIL_BINS="$MAIL_BIN $_MAIL_BINS"
-  fi
-
-  for b in $_MAIL_BINS; do
+  for b in $MAIL_BIN sendmail ssmtp mutt mail msmtp; do
     if _exists "$b"; then
       _MAIL_BIN="$b"
       break