Use YAML config for Appveyor settings, rather than their UI.

These take precedence over the UI, and I'm not sure the UI is taking
effect at the moment.

We don't need to build branches which start with 'dev-bundle-' since
those dev bundles won't be built yet when the tests are kicked off.

appveyor.yml

@@ -1,6 +1,14 @@
 version: '{build}'
+
+branches:
+  except:
+    - /^dev-bundle-/
+
+skip_branch_with_pr: true
+
 clone_folder: C:\projects\meteor
-os: Visual Studio 2015
+image: Visual Studio 2015
+
 environment:
   METEOR_PRETTY_OUTPUT: 0
   SELF_TEST_TOOL_NODE_FLAGS: " "