Fix the conda build on CircleCI

[bouweandela]

The conda build on CircleCI is failing all the time. I suspect this happens because CircleCI reports the wrong number of CPU cores.

Note: Java, Erlang and any other languages that introspect the /proc directory for information about CPU count may require additional configuration to prevent them from slowing down when using the CircleCI 2.0 resource class feature. Programs with this issue may request 32 CPU cores and run slower than they would when requesting one core.

from https://circleci.com/docs/2.0/configuration-reference/

This pull request limits the number of CPU cores used by the conda build to just 2.

[valeriupredoi]

cheers dude! I would not have known this - I don't really speak R. I triggered the run one more time so we can see it don't fail, then it's good to go - we should put this in the release (even if we froze it heh)

[valeriupredoi]

huzzah! 🎉