Use the original `ls` instead of aliases.

Fixes #755

bash_completion

@@ -57,7 +57,7 @@ __nvm_aliases ()
   declare aliases
   aliases=""
   if [ -d $NVM_DIR/alias ]; then
-    aliases="`cd $NVM_DIR/alias && ls`"
+    aliases="`cd $NVM_DIR/alias && command ls`"
   fi
   echo "${aliases}"
 }