tox: simplify with new-ish tox features

use 'extras' and 'usedevelop' instead of 'deps = --editable=.[dev]'

tox.ini

@@ -6,11 +6,12 @@
 [tox]
 envlist = {py27,py33,py34,py35,py36,pypy}
 skip_missing_interpreters = True
+minversion = 2.4.0
 
 [testenv]
-skip_install = True
+usedevelop = True
+extras = dev
 deps =
-    --editable=.[dev]
     pyflakes >= 1.2.3
 commands =
     pyflakes setup.py src
@@ -28,7 +29,6 @@ commands =
 
 [testenv:coverage]
 deps =
-    --editable=.[dev]
     pyflakes >= 1.2.3
     coverage
 commands =