Fix for new Xunit reporter options params

It looks like the new options implementation on the Xunit reporter brakes since we are calling it from mocha-multi without an options object. 
The new reporter is assuming that the (optional, optiosn) is always coming in and so it just brakes spectacularly. :-)

This fix pass an empty options as the second parameter to the reporters. 

The relevant commit is: https://github.com/Betterez/mocha-multi/commit/69e738bf513dfaabb9b032331459b61f3b31f6b8

Let me know what you think.

Cheers, Hernan.