[WIP] Trying to solve conda build problem

Created by: scarlehoff

My current guess is that some dependency issue is forcing conda-build to take the libraries from conda-forge while the compilers are from default: https://github.com/NNPDF/nnpdf/issues/1279#issuecomment-858396909

Also, the default conda forge is actually 2.17 so we are specifically getting an older one.

Going back to builds that worked, with gcc 9.1, nothing was coming from conda-forge which I would argue is generally a good thing: https://travis-ci.com/github/NNPDF/nnpdf/jobs/512446292#L385

Probably the actual solution would be to find which packages are bringing that dependency on 2.12, this commit is just to test whether this fixes it (because my guess would be that either lhapdf, or apfel or someone else will not work due to not having the correct libraries as well).

Btw, if I cancel the job for Mac in Travis do they all cancel?