Fix tmux reporting as screen

I forgot that Tmux copies Screen and modifies $TERM, which means that
both would return from _lp_multiplexer() as "screen". Move the check for
Tmux first so that it takes precedence.

liquidprompt

@@ -868,12 +868,12 @@ _lp_chroot() {
 }
 
 _lp_multiplexer() {
-    if [[ "$TERM" == screen* ]]; then
-        lp_mulitplexer=screen
-        return 0
-    elif [[ -n ${TMUX-} ]]; then
+    if [[ -n ${TMUX-} ]]; then
         lp_mulitplexer=tmux
         return 0
+    elif [[ "$TERM" == screen* ]]; then
+        lp_mulitplexer=screen
+        return 0
     fi
     return 1
 }