Now, with conda 4.1 packages are sorted by channel, version, build
number in decreasing priority. The highest matching package is
taken. https://conda.io/docs/user-guide/tasks/manage-channels.html
For the artiq-dev environment, the m-labs/label/dev channel should be
first, then the main channel, then defaults, and then conda-forge
(community supported packages).
closes#864
