Prevent maximum recursion RuntimeError caused by Tox

[hugovk]

Travis CI has started failing -- not due to Last.fm's APIs being broken after they relaunched ("re-imagined") their site -- but due to RuntimeError: maximum recursion depth exceeded. For example:

  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/tox/config.py", line 913, in _replace_env
    env_list = self.getdict('setenv')
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/tox/config.py", line 839, in getdict
    s = self.getstring(name, None)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/tox/config.py", line 893, in getstring
    x = self._replace(x)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/tox/config.py", line 993, in _replace
    return RE_ITEM_REF.sub(self._replace_match, x)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/tox/config.py", line 989, in _replace_match
    return handler(match)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/tox/config.py", line 913, in _replace_env
    env_list = self.getdict('setenv')
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/tox/config.py", line 839, in getdict
    s = self.getstring(name, None)
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/tox/config.py", line 880, in getstring
    x = self._cfg[s][name]
RuntimeError: maximum recursion depth exceeded

This happens for all Python versions. One of the Python packages that has changed since this last worked is Tox, which is now at 2.2.1. The last working build used Tox 2.1.1. So let's fix Tox at 2.1.1 (and later report this to Tox or Travis CI).

[hugovk]

Tox already know about their bug and have a fix underway: https://bitbucket.org/hpk42/tox/issues/285/tox-220-breaks-some-toxini-config-files