Old content will render to `-python=2.5`
This patch correct it to `--python=2.5`

See history of [bskinn/virtualenv:all-prompts branch](https://github.com/bskinn/virtualenv/commits/all-prompts), tip at
bskinn/virtualenv@67cda5f, for the actual sequence of
development of the prompt tests, leading to the first commit of the PR (e6a2b99)

* Added activate.ps1 to list of generated activation scripts
* extended tests to cover cmdline
* Use path_locations to test directories
* Add tests for powershell activation
* Fix environment creation for cmdline tests
* pwsh.exe on Windows
* make executable dependent tests optional locally
* fixes
* fix path normalization and casing
* turn path to long
* debug
* fix documentation

* Add a note, that bootstrap script can't install pip.

* Update reference.rst

Better wording from: https://github.com/pypa/virtualenv/pull/657#issuecomment-58411868

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.