Add AppVeyor fast-fail snippet

to slightly mitigate #3

Author: tkelman

appveyor.yml

@@ -20,6 +20,11 @@ notifications:
     on_build_status_changed: false
 
 install:
+# If there's a newer build queued for the same PR, cancel this one
+  - ps: if ($env:APPVEYOR_PULL_REQUEST_NUMBER -and $env:APPVEYOR_BUILD_NUMBER -ne ((Invoke-RestMethod `
+        https://ci.appveyor.com/api/projects/$env:APPVEYOR_ACCOUNT_NAME/$env:APPVEYOR_PROJECT_SLUG/history?recordsNumber=50).builds | `
+        Where-Object pullRequestId -eq $env:APPVEYOR_PULL_REQUEST_NUMBER)[0].buildNumber) { `
+        throw "There are newer queued builds for this pull request, failing early." }
 # Download most recent Julia Windows binary
   - ps: (new-object net.webclient).DownloadFile(
         $("http://s3.amazonaws.com/"+$env:JULIAVERSION),