Travis: upgrade tox: 2.3.1 => 2.7.0 (#488)

Author: blueyed

.travis.yml

@@ -80,7 +80,7 @@ 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==2.3.1
+  - pip install tox==2.7.0
 
 script:
   - tox