Update mixpanel adapter tracking code

[nselikoff]

No description provided.

[jherdman]

@nselikoff are you interested in making these changes still? If so, do you mind rebasing against master?

[jherdman]

#254 is likely related.

[nselikoff]

@jherdman I rebased against master and double checked that the snippet matches the latest from the mixpanel docs. The tests run and the dummy app looks good (though FYI, I had to temporarily change the token to my own to test the dummy app).

Also, I happened to notice that for folks implementing Mixpanel in the EU, there's a different config required: https://developer.mixpanel.com/docs/implement-mixpanel#implementing-mixpanel-in-the-european-union-eu

I'll create a new issue for this if there isn't one already.

[jherdman]

I'll create a new issue for this if there isn't one already.

That'd be great!