Update get-pip.py script URL on AppVeyor

Update also the way get-pip.py script is downloaded.

appveyor.yml

@@ -6,9 +6,9 @@ environment:
 install:
   - git submodule update --init --recursive
   - ps: $env:python = if ($env:arch -eq 32) { 'C:\Python27' } else { 'C:\Python27-x64' }
-  - ps: (new-object net.webclient).DownloadFile('https://raw.github.com/pypa/pip/master/contrib/get-pip.py', 'C:\get-pip.py')
+  - appveyor DownloadFile https://bootstrap.pypa.io/get-pip.py
   - set PATH=%python%;%python%\Scripts;%PATH%
-  - python C:\get-pip.py
+  - python get-pip.py
   - pip install -r python\test_requirements.txt
 build_script:
   - python run_tests.py