Remove pip --download-cache flag (fixes #279)

[cbenhagen]

From the pip changelog:

pip install --download-cache and pip wheel --download-cache command line flags have been deprecated and the functionality removed. Since pip now automatically configures and uses it's internal HTTP cache which supplants the --download-cache the existing options have been made non functional but will still be accepted until their removal in pip v8.0. For more information please see https://pip.pypa.io/en/stable/reference/pip_install.html#caching

[akshayaurora]

@cbenhagen thanks

[george72]

Fixes #279 removes the --download-cache that is deprecated, solving the problem but...

The buildozer installed with sudo pip install buildozer does not have those changes yet, I had to change them manually.

What is needed to include those changes on the stable buildozer?