Defaults to the current code for Analytics with doubleclick enabled

Hello, first of all thanks for the gem!

I'm aware that it is possible to enable dc with the version 0.0.4. But had been a while since GA update it's tracking, and I think it would be nice to have it as the default now.

I've commented on the code that the same code used before to enable it could be used now to disable it. I've also added a post install message warning those who are updating the gem.

What do you think?

Thanks!