Only patch assembly versions on master

AppVeyor PR builds get version numbers like 2.1.0.166-xwuhkwjc which
don't work as assembly versions

Author: tathamoddie

appveyor.yml

@@ -7,11 +7,17 @@ version: 2.1.0.{build}
 pull_requests:
   do_not_increment_build_number: true
 
-dotnet_csproj:
-  patch: true
-  file: '**\*.csproj'
-  version: '{version}'
-  package_version: '{version}'
+for:
+-
+  branches:
+    only:
+      - master
+
+  dotnet_csproj:
+    patch: true
+    file: '**\*.csproj'
+    version: '{version}'
+    package_version: '{version}'
 
 before_build:
 - nuget restore