AppVeyor: skip building branches and 32-bit Windows (#39)

AppVeyor as configured is too slow, so make it do less:

* Prevent duplicate AppVeyor builds on pull requests https://github.com/appveyor/ci/issues/882

* Only allow AppVeyor to build updates branch, https://www.appveyor.com/docs/branches/#white--and-blacklisting

* Only build for 64-bit Windows (x64) to improve CI times; remove x86

appveyor.yml

@@ -1,9 +1,12 @@
 version: 1.0.{build}
 image: Visual Studio 2017
 clone_depth: 1
+skip_branch_with_pr: true
+branches:
+    only:
+        - updates
 platform:
   - x64
-  - x86
 build_script:
 - cmd: >-
     echo on