Only deploy from release branches

This is to stop nuget being littered with *master* builds with misleading version numbers.

Author: Jos Hickson

appveyor.yml

@@ -25,5 +25,4 @@ deploy:
     secure: CxMH3k6k/Rpz1zjToGDAelLWDT/gDqRo08iEUAQsRnL+WRo+4TL5gLiNEUy/GfUy
   on:
     branch: 
-    - master
     - /release\/[0-9]\.[0-9]/