Appveyor - Move to VS 2017 image fully

Author: Liryna

appveyor.yml

@@ -12,27 +12,18 @@
       MSYS2_CACHE: '%DOKAN_CI_CACHE%\msys2'
       CHOCO_CACHE: '%DOKAN_CI_CACHE%\choco'
       WLK_INST_CACHE: '%DOKAN_CI_CACHE%\wlk_inst'
-      DOKAN_MAIN_BUILD_JOB_NAME: "Image: Visual Studio 2015; Configuration: All"
+      DOKAN_MAIN_BUILD_JOB_NAME: "Image: Visual Studio 2017; Configuration: All"
 
   version: 1.0.5-{build}
   configuration:
   - All
   - Coverity
   - FsTest
 
-  image:
-    - Visual Studio 2015
-    - Visual Studio 2017
+  image: Visual Studio 2017
 # If you suspect some changes on the AppVeyor-side breaking things,
 # uncomment below. https://www.appveyor.com/updates/
-#    - Previous Visual Studio 2015
 #    - Previous Visual Studio 2017
-  matrix:
-    exclude:
-      - configuration: All
-        image: Visual Studio 2017
-      - configuration: Coverity
-        image: Visual Studio 2017
   max_jobs: 1
   cache:
   - '%DOKAN_CI_CACHE% -> appveyor.yml'