ci: Fix appveyor build specifying correct AppVeyor image

Since scikit-build 0.10.0 more carefully tests availability of Visual Studio
generator, this commit fixes the issue using Visual Studio 2017 image
for python 3.6 and python 3.7

Author: jcfr

appveyor.yml

@@ -8,52 +8,62 @@ version: "0.0.1.{build}"
 environment:
   matrix:
 
-    - PYTHON_DIR: "C:\\Python27"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      PYTHON_DIR: "C:\\Python27"
       PYTHON_VERSION: "2.7.x"
       PYTHON_ARCH: "32"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python27-x64"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      PYTHON_DIR: "C:\\Python27-x64"
       PYTHON_VERSION: "2.7.x"
       PYTHON_ARCH: "64"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python34"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      PYTHON_DIR: "C:\\Python34"
       PYTHON_VERSION: "3.4.x"
       PYTHON_ARCH: "32"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python34-x64"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      PYTHON_DIR: "C:\\Python34-x64"
       PYTHON_VERSION: "3.4.x"
       PYTHON_ARCH: "64"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python35"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      PYTHON_DIR: "C:\\Python35"
       PYTHON_VERSION: "3.5.x"
       PYTHON_ARCH: "32"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python35-x64"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      PYTHON_DIR: "C:\\Python35-x64"
       PYTHON_VERSION: "3.5.x"
       PYTHON_ARCH: "64"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python36"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      PYTHON_DIR: "C:\\Python36"
       PYTHON_VERSION: "3.6.x"
       PYTHON_ARCH: "32"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python36-x64"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      PYTHON_DIR: "C:\\Python36-x64"
       PYTHON_VERSION: "3.6.x"
       PYTHON_ARCH: "64"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python37"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      PYTHON_DIR: "C:\\Python37"
       PYTHON_VERSION: "3.7.x"
       PYTHON_ARCH: "64"
       BLOCK: "0"
 
-    - PYTHON_DIR: "C:\\Python37-x64"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      PYTHON_DIR: "C:\\Python37-x64"
       PYTHON_VERSION: "3.7.x"
       PYTHON_ARCH: "64"
       BLOCK: "0"