Fixed appveyor error

.appveyor.yml

@@ -38,9 +38,11 @@ before_build:
       cd build
       if ("$env:APPVEYOR_JOB_NAME" -match "Image: Visual Studio 2013") {
           $env:generator="Visual Studio 12 2013"
-      } else if ("$env:APPVEYOR_JOB_NAME" -match "Image: Visual Studio 2015") {
+      }
+      if ("$env:APPVEYOR_JOB_NAME" -match "Image: Visual Studio 2015") {
           $env:generator="Visual Studio 14 2015"
-      } else {
+      }
+      if ("$env:APPVEYOR_JOB_NAME" -match "Image: Visual Studio 2017") {
           $env:generator="Visual Studio 15 2017"
       }
       if ($env:PLATFORM -eq "x64") {