CI: fix conda version on appveyor

Author: jreback

appveyor.yml

@@ -65,7 +65,8 @@ install:
 
   # install our build environment
   - cmd: conda config --set show_channel_urls true --set always_yes true --set changeps1 false
-  - cmd: conda update -q conda
+  #- cmd: conda update -q conda
+  - cmd: conda install conda=4.2.15
   - cmd: conda config --set ssl_verify false
 
   # add the pandas channel *before* defaults to have defaults take priority