Require the latest tox version

Hong Minhee · dahlia · commit dc13fd9a7bc3 · 2018-04-13T02:38:37.000+09:00
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -79,6 +79,6 @@ dependencies):
  -  Python
      -  All Python versions that Nirum should support: Python 2.7, and
         3.4 and above all
-     -  [`tox`][tox]
+     -  [`tox`][tox] 3.0.0 or higher
 
 [tox]: https://tox.readthedocs.io/
diff --git a/tox.ini b/tox.ini
@@ -1,4 +1,5 @@
 [tox]
+minversion = 3.0.0
 envlist =
     buildfixture
     # CHECK: When the list of supported Python versions change,

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,5 @@`
`1`	`1`	`[tox]`
	`2`	`+minversion = 3.0.0`
`2`	`3`	`envlist =`
`3`	`4`	`buildfixture`
`4`	`5`	`# CHECK: When the list of supported Python versions change,`