AppVeyor: Use the new location for the get-pip.py script

Since the existing location fails with:
```
Running Install scripts
(new-object net.webclient).DownloadFile('https://raw.github.com/pypa/pip/master/contrib/get-pip.py', 'C:/get-pip.py')
%PYTHON%/python.exe C:/get-pip.py
You're using an outdated location for the get-pip.py script, please use the one available from https://bootstrap.pypa.io/get-pip.py
Command exited with code 1
```

appveyor.yml

@@ -9,7 +9,7 @@ init:
   - "ECHO %PYTHON%"
   - ps: "ls C:/Python*"
 install:
-  - ps: (new-object net.webclient).DownloadFile('https://raw.github.com/pypa/pip/master/contrib/get-pip.py', 'C:/get-pip.py')
+  - ps: (new-object net.webclient).DownloadFile('https://bootstrap.pypa.io/get-pip.py', 'C:/get-pip.py')
   - "%PYTHON%/python.exe C:/get-pip.py"
   - "%PYTHON%/Scripts/pip.exe install -e ."
 test_script: