Push artefacts with powershell rather than CLI

It looks like there are bugs with the appveyor CLI. The work around is
to use the powershell cmdlets directly.

appveyor.yml

@@ -48,7 +48,7 @@ install:
     )
   - if "%compiler_name%"=="gcc" (@set "mingw_log_folder=%mingw_output_folder%\logs")
   - if exist "%mingw_log_folder%" @for /f %%f in ('dir /b /oD /tc "%mingw_log_folder%"') do @set "mingw_log_file=%mingw_log_folder%\%%f"
-  - if exist "%mingw_log_file%" appveyor PushArtifact "%mingw_log_file%" -FileName mingw-download.log
+  - if exist "%mingw_log_file%" powershell Push-AppveyorArtifact "%mingw_log_file%" -FileName mingw-download.log
 
 before_build:
   # Set up mingw commands
@@ -99,7 +99,7 @@ before_build:
     -DCIVETWEB_C_STANDARD=%c_standard%
     -DCIVETWEB_CXX_STANDARD=%cxx_standard%
     "%source_path%"
-  - appveyor PushArtifact CMakeCache.txt
+  - powershell Push-AppveyorArtifact CMakeCache.txt
   - cd "%source_path%"
 
 build_script: