tools/install.sh: catch up oftest change

Since oftest removed tools/munger directory, install.sh fails as follows
> mininet/util/install.sh: line 399: cd: tools/mnuger: No such file or directory

oftest change set
> commit be8503a69d609d0aee844a91f3f5d66f4e2666c7
> Author: Rich Lane <rlane@bigswitch.com>
> Date:   Tue Mar 12 10:16:33 2013 -0700
>
>     remove pylibopenflow tools

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>