Added check for 'Rank' on Auxiliary modules

See @jvazquez-r7 comment here:
https://github.com/rapid7/metasploit-framework/pull/3070#discussion_r11082744

This addition checks if the module is an aux module and if there is a 'Rank = ' line.

Example output:
```
firefart@LinuxMint ~/Coding/metasploit-framework $ ruby tools/msftidy.rb modules/ | grep Aux
modules/auxiliary/dos/hp/data_protector_rds.rb:9 - [WARNING] Auxiliary modules have no 'Rank': Rank = ManualRanking
modules/auxiliary/dos/tcp/junos_tcp_opt.rb:14 - [WARNING] Auxiliary modules have no 'Rank': Rank = ManualRanking
modules/auxiliary/dos/windows/smb/ms11_019_electbowser.rb:7 - [WARNING] Auxiliary modules have no 'Rank': Rank = ManualRanking
modules/auxiliary/gather/mantisbt_admin_sqli.rb:9 - [WARNING] Auxiliary modules have no 'Rank': Rank = GoodRanking
```