Add workaround for canceling pending build jobs

As suggested here
https://github.com/appveyor/ci/issues/38#issuecomment-70628826

appveyor.yml

@@ -14,6 +14,10 @@ environment:
     - VSVER: 10
 
 install:
+  - 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." }
   - date /T & time /T
   - set PATH=C:\cygwin\bin;%PATH%
   - set OPENSSL_VER=1_0_2d