Fix failed AppVeyor web requests (#309)

Allow we requests to servers that only support TLS 1.2. Previously
failed with the error:

  Exception calling "DownloadFile" with "2" argument(s): "The request
  was aborted: Could not create SSL/TLS secure channel."

Was marking all contributions as broken which is not helpful for
reviewing.

Fixes #308

Author: jdufresne

.appveyor.yml

@@ -1,5 +1,7 @@
 # To activate, change the Appveyor settings to use `.appveyor.yml`.
 install:
+  - ps: "[Net.ServicePointManager]::SecurityProtocol = [System.Net.SecurityProtocolType]::Tls12"
+
   - python -m pip install --upgrade virtualenv pip setuptools tox
 
   # Fetch the three main PyPy releases