change image type

Author: chris-b1

appveyor.yml

@@ -20,18 +20,14 @@ environment:
   matrix:
 
     - CONDA_ROOT: "C:\\Miniconda3_64"
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       PYTHON_VERSION: "3.6"
       PYTHON_ARCH: "64"
       CONDA_PY: "36"
       CONDA_NPY: "113"
 
     - CONDA_ROOT: "C:\\Miniconda3_64"
-      PYTHON_VERSION: "3.7"
-      PYTHON_ARCH: "64"
-      CONDA_PY: "37"
-      CONDA_NPY: "114"
-
-    - CONDA_ROOT: "C:\\Miniconda3_64"
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       PYTHON_VERSION: "2.7"
       PYTHON_ARCH: "64"
       CONDA_PY: "27"