[openmm] 7.4.2 rc

[jchodera]

This PR attempts to build OpenMM 7.4.2 rc

cc: openmm/openmm#2673

[jchodera]

I'm not quite sure why the osx builds are failing in devtools/osx-build.sh:

+MACOSX_DEPLOYMENT_TARGET=10.13
+curl -s -O https://repo.continuum.io/miniconda/Miniconda3-4.6.14-MacOSX-x86_64.sh
+bash Miniconda3-4.6.14-MacOSX-x86_64.sh -b -p /Users/travis/anaconda
+export PATH=/Users/travis/anaconda/bin:/Users/travis/.rvm/gems/ruby-2.6.3/bin:/Users/travis/.rvm/gems/ruby-2.6.3@global/bin:/Users/travis/.rvm/rubies/ruby-2.6.3/bin:/Users/travis/.rvm/bin:/Users/travis/bin:/Users/travis/.local/bin:/Users/travis/.nvm/versions/node/v13.0.1/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin
+PATH=/Users/travis/anaconda/bin:/Users/travis/.rvm/gems/ruby-2.6.3/bin:/Users/travis/.rvm/gems/ruby-2.6.3@global/bin:/Users/travis/.rvm/rubies/ruby-2.6.3/bin:/Users/travis/.rvm/bin:/Users/travis/bin:/Users/travis/.local/bin:/Users/travis/.nvm/versions/node/v13.0.1/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin
+conda config --add channels conda-forge
devtools/osx-build.sh: line 20: conda: command not found

Any ideas @jaimergp?

[jaimergp]

I had forgotten how slow Travis can be... Let's wait. I think the conda init hooks should enable the command in a less hacky way, but this is also a deliberately outdated Miniconda and then we are going even further back to pin conda build. We'll see. It might not work at all.

[jaimergp]

Conda Build 2 is so old is not even on defaults anymore. Fortunately, there's a copy on conda-forge. But when the last CB2 was out, Python 3.7 hadn't been released, so there's no packages for that Python version. This can go and on and on... are we sure we want to do it this way? I'd rather invest the time if leveraging conda-dev-recipes for this release. Next one will hopefully use conda-forge.

[jchodera]

We need to get this release out ASAP, so we don't have time to wait for the conda-forge issues to be sorted out (as much as I think that's absolutely critical).

[jaimergp]

Yes, I agree CF cannot be part of this release. I meant using the pipelines at conda-dev-recipes (no conda-forge involved).

[jchodera]

Just merged #24, so we don't need this fix.