Revert "AppVeyor: Update to Visual Studio 2017"

This reverts commit 9f51db7.

Builds fails currently with Visual Studio 2017.

Author: Croydon

appveyor.yml

@@ -1,7 +1,7 @@
 # appveyor file
 # https://www.appveyor.com/docs/appveyor-yml/
 
-image: Visual Studio 2017
+image: Visual Studio 2015
 
 platform:
   - x64
@@ -23,7 +23,7 @@ init:
 environment:
   JOBS: 4
   GIT_SSH: c:\projects\nodegit\vendor\plink.exe
-  GYP_MSVS_VERSION: 2017
+  GYP_MSVS_VERSION: 2015
   matrix:
     # Node.js
     - nodejs_version: "10"