Merge pull request #1877 from tseaver/appveyor-grpc-issue-6939-wag

WAG:  try to preinstall 'grpcio' on appveyor.

appveyor.yml

@@ -30,10 +30,12 @@ environment:
     - PYTHON: "C:\\Python27"
       PYTHON_VERSION: "2.7.11"
       PYTHON_ARCH: "32"
+      INSTALL_GRPC: "1"
 
     - PYTHON: "C:\\Python27-x64"
       PYTHON_VERSION: "2.7.11"
       PYTHON_ARCH: "64"
+      INSTALL_GRPC: "1"
 
     # Python 3.4.4 is the latest Python 3.4 with a Windows installer
     # Python 3.4.4 is the overall latest
@@ -87,6 +89,9 @@ install:
   # target Python version and architecture
   - "%CMD_IN_ENV% pip install wheel nose nose-exclude unittest2 cryptography"
 
+  # Work around https://github.com/grpc/grpc/issues/6939
+  - '%CMD_IN_ENV% if "%INSTALL_GRPC%"=="1" (pip install grpcio)'
+
 build_script:
   # Build the compiled extension
   - "%CMD_IN_ENV% python setup.py build"