solve issue powershell

Author: glemaitre

Diff for: appveyor.yml

@@ -38,7 +38,7 @@ install:
   # https://github.com/conda/conda/issues/1753
   - "SET PATH=%PYTHON%;%PYTHON%\\Scripts;%PYTHON%\\Library\\bin;%PATH%"
   - conda update conda -y -q
-  - conda create -n testenv --yes python=$PYTHON_VERSION pip
+  - conda create -n testenv --yes python=%PYTHON_VERSION% pip
   - activate testenv
   - conda install scipy numpy -y -q
   - pip install --pre scikit-learn