Add another testcase for matching a range of browser versions for BES

Instead of specifying one particular version, the BrowserExploitServer mixin now also supports matching a range of versions using Proc. Need to add this to rspec.

Test:

```
$ rspec spec/lib/msf/core/exploit/remote/browser_exploit_server_spec.rb 
Msf::Exploit::Remote::BrowserExploitServer ..................

Finished in 3.02 seconds
18 examples, 0 failures
```