add alias to anaconda folder

[freeman-lab]

When the base image was last updated, and the version of anaconda was updated, a folder previously named anaconda was renamed anaconda2, see here

https://github.com/binder-project/binder-build-core/blob/master/images/base/Dockerfile#L46

We didn't think that anyone was directly referencing this folder, but some users were (e.g. @gcr). For backwards compatibility, we should update the image with an alias that points anaconda to what is now anaconda2.

@andrewosh is that correct? @sethvincent want to take a try at this?

[sethvincent]

Yes, I can take a look at this.