Disabled the NuGet symbols deploy

Apparently the SSL certificate for SymbolSource has expired, throwing a
build error for our NuGet deploy.
 re: NuGet/Home#6082 (comment)

appveyor.yml

@@ -46,6 +46,7 @@ deploy:
     # NuGet Deployment for releases
   - provider: NuGet
     server: 
+    skip_symbols: true
     api_key:
       secure: vEophXSqus5F60LRBY4/j1l6K/S5+n3/yYpiID3O7JJW1gyj+0q0enuHhN3tgdhl
     artifact: /.*\.nupkg/