AppVeyor: Conda-Forge Installs

Use the conda-forge packages instead of the default packages.
Especially for numpy, this means using OpenBLAS instead of MKL.

conda-forge/numpy-feedstock#84
conda-forge/numpy-feedstock#97

.appveyor.yml

@@ -107,11 +107,11 @@ install:
   # Add our channels.
   - cmd: conda config --set show_channel_urls true
   - cmd: conda config --remove channels defaults
-  - cmd: conda config --add channels defaults
   - cmd: conda config --add channels conda-forge
+  - cmd: conda config --add pinned_packages conda-forge::numpy
 
   # Configure the VM.
-  - cmd: conda install -n root --quiet --yes cmake hdf5 numpy
+  - cmd: conda install -n root --quiet --yes -c conda-forge numpy cmake hdf5
 
 before_build:
   - cmd: cd C:\projects\openpmd-api