try using stable version of gitverison

Author: Scott Williams

appveyor.yml

@@ -4,7 +4,7 @@ init:
   - git config --global core.autocrlf true  # otherwise we have newline errors
 
 install:
-  - choco install gitversion.portable -pre -y
+  - choco install gitversion.portable -y
 
 before_build:
   - cmd: dotnet --version