Pin tox version to 2.1.1 to workaround tox issue pytest-dev#285.

See
https://bitbucket.org/hpk42/tox/issues/285/tox-220-breaks-some-toxini-config-files
for more details.

Author: pelme

.travis.yml

@@ -55,5 +55,5 @@ install:
   - sed -i.bak 's/whitelist_externals =/\0\n    travis_retry_pip/' tox.ini
   - diff tox.ini tox.ini.bak && return 1 || true
 
-  - pip install tox
+  - pip install tox==2.1.1
 script: tox -e $TESTENV

generate_configurations.py

@@ -253,7 +253,7 @@ def make_travis_yml(envs):
           - sed -i.bak 's/whitelist_externals =/\\0\\n    travis_retry_pip/' tox.ini
           - diff tox.ini tox.ini.bak && return 1 || true
 
-          - pip install tox
+          - pip install tox==2.1.1
         script: tox -e $TESTENV
         """).strip("\n")
     testenvs = '\n'.join('  - TESTENV=%s' % testenv_name(env) for env in envs)