Update tox to 2.5.0

[pyup-bot]

There's a new version of tox available.
You are currently using 2.4.1. I have updated it to 2.5.0

These links might come in handy: PyPI | Changelog | Docs

Changelog

2.5.0

---

• slightly backward incompatible: fix issue310: the {posargs} substitution
  now properly preserves the tox command line positional arguments. Positional
  arguments with spaces are now properly handled.
  NOTE: if your tox invocation previously used extra quoting for positional arguments to
  work around issue310, you need to remove the quoting. Example:
  tox -- "'some string'" has to now be written simply as
  tox -- "some string"
  thanks holger krekel. You can set minversion = 2.5.0 in the [tox]
  section of tox.ini to make sure people using your tox.ini use the correct version.

• fix 359: add COMSPEC to default passenv on windows. Thanks
  anthrotype.

• add support for py36 and py37 and add py36-dev and py37(nightly) to
  travis builds of tox. Thanks John Vandenberg.

• fix 348: add py2 and py3 as default environments pointing to
  "python2" and "python3" basepython executables. Also fix 347 by
  updating the list of default envs in the tox basic example.
  Thanks Tobias McNulty.

• make "-h" and "--help-ini" options work even if there is no tox.ini,
  thanks holger krekel.

• add {:} substitution, which is replaced with os-specific path
  separator, thanks Lukasz Rogalski.

• fix 305: downloadcache test env config is now ignored as pip-8
  does caching by default. Thanks holger krekel.

• output from install command in verbose (-vv) mode is now printed to console instead of
  being redirected to file, thanks Lukasz Rogalski

• fix 399. Make sure {envtmpdir} is created if it doesn't exist at the
  start of a testenvironment run. Thanks Manuel Jacob.

• fix 316: Lack of commands key in ini file is now treated as an error.
  Reported virtualenv status is 'nothing to do' instead of 'commands
  succeeded', with relevant error message displayed. Thanks Lukasz Rogalski.

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #28