Use tox-travis

dc9cd42 introduced Travis checking of flakes using tox.
However tox.ini only defined 'flakes-py34', so Travis
was only invoking flake8 on Python 3.4, and invoking
py.test on other Python versions.

As tox factors only work properly on the default testenv,
use a generic tox environment 'flakes', and use tox-travis
to select the correct basepython.

.travis.yml

@@ -37,9 +37,7 @@ python:
 - 3.4
 - pypy
 install:
-- pip install tox
+- pip install tox-travis
 - if [[ $TOX_SUFFIX != flakes ]]; then python setup.py install ; fi
 script:
-- v=$TRAVIS_PYTHON_VERSION
-- if [[ ${v%py} != $v ]]; then TOXPY=$v; else TOXPY=py"${v/./}"; fi
-- tox -e "${TOXPY}-${TOX_SUFFIX}"
+- tox -e "${TOX_SUFFIX}"