Replaced pypi.python.org with pypi.org in the download URL.
Added `-L` option to `curl` to follow redirects from randomized hash
URLS.

Also, replaced pypi.python.org/pypi with pypi.org/project throughout the
docs.

This fix addresses the most important part of #1042.

* Will only accept Wheels (because that is all we can install
  without setuptools).
* Will still fall back to using the bundled copies of the
  preinstalled packages if it cannot access the internet.

This should prevent errors happening from too old of versions of
software being bundled with virtualenv, while still allowing people
to prevent talking to the internet with --no-download.