Update pip for SCons installation on AppVeyor

Python 2.7.11 seems to ship with pip 7.1.2, which fails to pip install scons:

pip --version
pip 7.1.2 from c:\python27\lib\site-packages (python 2.7)
pip install scons
Collecting scons
  Downloading scons-2.5.0.tar.gz (620kB)
Installing collected packages: scons
  Running setup.py install for scons
    Complete output from command c:\python27\python.exe -c "import setuptools, tokenize;__file__='c:\\users\\appveyor\\appdata\\local\\temp\\1\\pip-build-g7cqps\\scons\\setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record c:\users\appveyor\appdata\local\temp\1\pip-a8r4ou-record\install-record.txt --single-version-externally-managed --compile:
    usage: -c [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
       or: -c --help [cmd1 cmd2 ...]
       or: -c --help-commands
       or: -c cmd --help

    error: option --single-version-externally-managed not recognized

    ----------------------------------------
Command "c:\python27\python.exe -c "import setuptools, tokenize;__file__='c:\\users\\appveyor\\appdata\\local\\temp\\1\\pip-build-g7cqps\\scons\\setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record c:\users\appveyor\appdata\local\temp\1\pip-a8r4ou-record\install-record.txt --single-version-externally-managed --compile" failed with error code 1 in c:\users\appveyor\appdata\local\temp\1\pip-build-g7cqps\scons
You are using pip version 7.1.2, however version 8.1.1 is available.
You should consider upgrading via the 'python -m pip install --upgrade pip' command.
Command exited with code 1

https://ci.appveyor.com/project/GodotBuilder/godot-builds/build/1.0.3/job/99s7raxfmm780i3s#L7

The solution is to upgrade pip to latest version before installing scons.

GodotBuilder / godot-builds

Update pip for SCons installation on AppVeyor #2