Fix AppVeyor build (#1241)

Don't use pre-release version of GitVersion since that fails
on AppVeyor. Instead hard-code version 5.11.1 which is the one
that previously worked.

Author: ermshiperete

appveyor.yml

@@ -9,7 +9,7 @@ init:
       set GitVersion_NoNormalizeEnabled=true
       set IGNORE_NORMALISATION_GIT_HEAD_MOVE=1
 install:
-  - choco install gitversion.portable -pre -y
+  - choco install gitversion.portable --version 5.11.1 -y
 nuget:
   disable_publish_on_pr: true
   disable_publish_octopus: true