Actually test alternate rubies.

Looks like this is used so infrequently that the check wouldn't have worked anyway.

Now, it does.

````
$ MSF_CHECK_OLD_RUBIES=true tools/msftidy.rb modules/auxiliary/scanner/rdp/ms12-020_check.rb
This is going to take a while, depending on the number of Rubies you have installed.
Checking syntax for ms12-020_check.rb.
ms12-020_check.rb:37: syntax error, unexpected ')'
ms12-020_check.rb - [ERROR] Fails alternate Ruby version check

````