Update conda installation instructions

[KennellyT]

The pyne installation instructions for conda on pyne.io are:
conda install -c cyclus -c pyne pyne=VERSION
where they should be:
'conda install -c conda-forge -c pyne pyne=VERSION` as noted in Downloading Pyne #1012 .

I would also recommend that the VERSION number '0.5.11' should be noted in the instructions as the statement in the instructions:

where VERSION should be replaced with the version number to be installed

is vague as I found difficulty finding the version numbers of pyne.
Thank you

[scopatz]

Thanks for pointing out this issue with the docs! Yeah, those install instructions are out of date. You don't need version at all. The real install instructions should just be:

$ conda install -c conda-forge pyne

We have fixed this in the README: https://github.com/pyne/pyne#binary

[KennellyT]

Thank you.
I have also edited the installation script via conda (conda.html) for Pyne.io in a pull reuqest.