Utilize tox in travis-ci

Utilize tox in travis-ci to simplify its configuration.

Signed-off-by: Satoru SATOH <satoru.satoh@gmail.com>

.travis.yml

@@ -12,17 +12,9 @@ env:
   - REMOVE_LOCALES=false
   - REMOVE_LOCALES=true
 install:
-  - pip install pyyaml coveralls flake8 flake8-import-order doc8
-  - if [[ $TRAVIS_PYTHON_VERSION != 2* ]]; then pip install sphinx; fi
-  - pip install .
+  - pip install tox-travis
   - if [[ $REMOVE_LOCALES = "true" ]]; then sudo rm -rf /usr/lib/locale/*; fi
 script:
-  - if [[ $TRAVIS_PYTHON_VERSION != nightly ]]; then flake8 .; fi
-  - if [[ $TRAVIS_PYTHON_VERSION != 2* ]]; then doc8 $(git ls-files '*.rst'); fi
-  - yamllint --strict $(git ls-files '*.yaml' '*.yml')
-  - coverage run --source=yamllint setup.py test
-  - if [[ $TRAVIS_PYTHON_VERSION != 2* ]]; then
-      python setup.py build_sphinx;
-    fi
+  - tox
 after_success:
   coveralls