move 6to5ify from dev-dependencies to dependencies

I think `6to5ify` needs to be a dependency and not a dev dependency.

[As seen in this simple example](http://requirebin.com/?gist=d5a805ecc141fbfd5129), if you require `react-geocoder` console it out, you get an error:

![](https://cldup.com/4NaoGgh5Fk.png) 

cc @tmcw