Change max LogLevel to 3

For a long time, we've been telling people to "setg LogLevel 5" for loggnig/debugging purposes. But if you look at how the log levels are defined in rex/constants.rb, you will see there are only 4 log levels: LEV_0, LEV_1, LEV_2, and LEV_3.

## To verify this claim:

 - [ ] Look at rex/constants.rb and see it for yourself: https://github.com/rapid7/metasploit-framework/blob/master/lib/rex/constants.rb#L13

- [ ] You can also test this by first adding a ```1/0``` to an existing module's exploit method. And then you do ```setg LogLevel 5```, do ```tail -f ~/.msf4/logs/framework.log```, and you should never see a log level 5 message.

Note: if you don't know how to read the log, the format looks like this:
[date] [log level] [source] [error]

Take the following error for example:

```
[09/26/2014 14:20:28] [e(0)] core: Exploit failed (windows/browser/ie_cbutton_uaf): ZeroDivisionError divided by 0
```

e(0) means this is log level 0.