Add second condition for determining Windows platform

You're currently using 
```ruby 
RbConfig::CONFIG['host_os'] =~ /win32/
```
to identify when the gem is being used on Windows. This works for JRuby on Windows 7, which results to ```mswin32```. But on Windows 7 and MRI 1.9.3, this results to ```mingw32```. 

My PR just adds ```/mingw32/``` as a condition for identifying the Windows platform.