BLD: add in build conflict resolution to appeveyor.yml

Author: jreback

appveyor.yml

@@ -49,6 +49,12 @@ init:
   - "ECHO %PYTHON_VERSION% %PYTHON%"
 
 install:
+  # cancel older builds for the same PR
+  - 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." }
+
   # this installs the appropriate Miniconda (Py2/Py3, 32/64 bit)
   # updates conda & installs: conda-build jinja2 anaconda-client
   - powershell .\ci\install.ps1
@@ -69,6 +75,7 @@ install:
   # https://github.com/conda/conda-build/issues/1001
   # disabling 3.4 as windows complains upon compiling byte
   # code
+
   - cmd: conda install conda-build=1.21.7
   - cmd: conda config --set ssl_verify false