Auto merge of #1929 - micbou:appveyor, r=puremourning

[READY] Update get-pip.py script URL on AppVeyor

This fixes the following error on AppVeyor:
```
You're using an outdated location for the get-pip.py script, please use the one available from https://bootstrap.pypa.io/get-pip.py
```

I also changed the way `get-pip.py` is downloaded. This is now done like in the `ycmd` repository.

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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