AppVeyor: explicitly select worker image

Currently, we're using the default Visual Studio 2015 image for
everything.

Closes https://github.com/curl/curl/pull/3606

appveyor.yml

@@ -2,42 +2,48 @@ version: 7.50.0.{build}
 
 environment:
     matrix:
-      - PRJ_GEN: "Visual Studio 9 2008"
+      - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
+        PRJ_GEN: "Visual Studio 9 2008"
         PRJ_CFG: Release
         OPENSSL: OFF
         WINSSL: ON
         HTTP_ONLY: OFF
         TESTING: OFF
         SHARED: ON
-      - PRJ_GEN: "Visual Studio 14 2015 Win64"
+      - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
+        PRJ_GEN: "Visual Studio 14 2015 Win64"
         PRJ_CFG: Release
         OPENSSL: OFF
         WINSSL: ON
         HTTP_ONLY: OFF
         TESTING: OFF
         SHARED: ON
-      - PRJ_GEN: "Visual Studio 14 2015 Win64"
+      - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
+        PRJ_GEN: "Visual Studio 14 2015 Win64"
         PRJ_CFG: Release
         OPENSSL: ON
         WINSSL: OFF
         HTTP_ONLY: OFF
         TESTING: OFF
         SHARED: ON
-      - PRJ_GEN: "Visual Studio 10 2010 Win64"
+      - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
+        PRJ_GEN: "Visual Studio 10 2010 Win64"
         PRJ_CFG: Debug
         OPENSSL: OFF
         WINSSL: OFF
         HTTP_ONLY: OFF
         TESTING: ON
         SHARED: OFF
-      - PRJ_GEN: "Visual Studio 14 2015 Win64"
+      - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
+        PRJ_GEN: "Visual Studio 14 2015 Win64"
         PRJ_CFG: Debug
         OPENSSL: OFF
         WINSSL: OFF
         HTTP_ONLY: OFF
         TESTING: ON
         SHARED: OFF
-      - PRJ_GEN: "Visual Studio 14 2015 Win64"
+      - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2015"
+        PRJ_GEN: "Visual Studio 14 2015 Win64"
         PRJ_CFG: Debug
         OPENSSL: OFF
         WINSSL: OFF