Fix appveyor config for pip

appveyor.yml

@@ -3,7 +3,7 @@ environment:
   matrix:
     - PYTHON: "C:/Python27"
 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"
 test_script:
   - "%PYTHON%/Scripts/pip.exe --version"