Support non-mingw32 windows

Previous windows support only checked for host_os of "mingw32" and a 32-bit host_cpu, this should make it a bit more comprehensive.
I think it would be worth while to override this method an part of the Windows platform block. Just changing the 'which' to 'where' work.

```ruby
def system_phantomjs_path
        `where phantomjs`.delete("\n")
rescue
end
```