AppVeyor: update to Visual Studio 2017

Switch all Visual Studio 2015 builds to Visual Studio 2017. It's not a
moving target anymore as the last update, Update 9, has been released.

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

appveyor.yml

@@ -11,7 +11,7 @@ environment:
         TESTING: OFF
         SHARED: ON
       - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2017"
-        PRJ_GEN: "Visual Studio 14 2015 Win64"
+        PRJ_GEN: "Visual Studio 15 2017 Win64"
         PRJ_CFG: Release
         OPENSSL: OFF
         WINSSL: ON
@@ -19,7 +19,7 @@ environment:
         TESTING: OFF
         SHARED: ON
       - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2017"
-        PRJ_GEN: "Visual Studio 14 2015 Win64"
+        PRJ_GEN: "Visual Studio 15 2017 Win64"
         PRJ_CFG: Release
         OPENSSL: ON
         WINSSL: OFF
@@ -35,7 +35,7 @@ environment:
         TESTING: ON
         SHARED: OFF
       - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2017"
-        PRJ_GEN: "Visual Studio 14 2015 Win64"
+        PRJ_GEN: "Visual Studio 15 2017 Win64"
         PRJ_CFG: Debug
         OPENSSL: OFF
         WINSSL: OFF
@@ -43,7 +43,7 @@ environment:
         TESTING: ON
         SHARED: OFF
       - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2017"
-        PRJ_GEN: "Visual Studio 14 2015 Win64"
+        PRJ_GEN: "Visual Studio 15 2017 Win64"
         PRJ_CFG: Debug
         OPENSSL: OFF
         WINSSL: OFF