Update .appveyor.yml

Author: martin-frbg

.appveyor.yml

@@ -22,7 +22,8 @@ install:
   - call %CONDA_INSTALL_LOCN%\Scripts\activate.bat
   - conda config --set auto_update_conda false
   - conda config --add channels conda-forge --force
-  - conda install --yes conda-forge/win-64::charset-normalizer --force  
+  - conda install conda=4.10.3
+  - conda install --yes charset-normalizer --force  
   - conda update --yes -n base conda
   - conda install --yes --quiet flang jom
   - call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" amd64