Include `install` and `uninstall` in help if ruby-build is installed

libexec/rbenv-help

@@ -93,8 +93,6 @@ print_summary() {
 
   if [ -n "$summary" ]; then
     printf "   %-9s   %s\n" "$command" "$summary"
-  else
-    return 1
   fi
 }
 
@@ -145,7 +143,7 @@ if [ -z "$1" ] || [ "$1" == "rbenv" ]; then
   [ -z "$usage" ] || exit
   echo
   echo "Some useful rbenv commands are:"
-  print_summaries commands rehash global local shell version versions which whence
+  print_summaries commands global local shell install uninstall rehash version versions which whence
   echo
   echo "See \`rbenv help <command>' for information on a specific command."
   echo "For full documentation, see: https://github.com/sstephenson/rbenv#readme"