Move activation into conda-env

This provides a backwards compatible method of moving activation over to [conda-env][].  It requires conda-env v1.2.0 to be installed, but once that's installed everything works as it did + the new activation behavior.

[conda-env]: https://github.com/conda/conda-env
Actually -- it doesn't.  This should only be displayed when someone is attempting to activate an environment from within the root environment.  I've added a note pointing people to the conda-env homepage.