[tox] Move flake8 into its own environment.

tox.ini

@@ -1,12 +1,12 @@
 [tox]
-envlist = py27, py35
+envlist = py27, py35, flake8
 
 [testenv]
 commands =
     {envbindir}/py.test --cov=dosage --cov=dosagelib --tb=short -n4 \
         --cov-report=xml:{toxworkdir}/cov-{envname}.xml --cov-report=term \
         --junitxml={toxworkdir}/junit-{envname}.xml {posargs}
-    - {envbindir}/flake8
+
 passenv = TRAVIS
 # Also install optional dependencies for tests.
 deps =
@@ -15,6 +15,11 @@ deps =
     pytest-travis-fold
     cssselect
     Pillow
+
+[testenv:flake8]
+commands =
+    - {envbindir}/flake8
+deps =
     flake8
     flake8-coding
     flake8-future-import