Switches to Qt 5.12 on AppVeyor

Author: j-ulrich

.appveyor.yml

@@ -4,7 +4,7 @@ environment:
     CTEST_OUTPUT_ON_FAILURE: '1'
     matrix:
         - CMAKE_GENERATOR: Visual Studio 15 2017 Win64
-          QT_DIR: C:\Qt\5.11\msvc2017_64
+          QT_DIR: C:\Qt\5.12\msvc2017_64
           APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
           VCVARS_COMMANDLINE: '"C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"'
         - CMAKE_GENERATOR: Visual Studio 14 2015