Fix tox -e coverage.

Closes #309. Thanks @pawl

Author: Julian

tox.ini

@@ -28,9 +28,10 @@ commands =
 deps =
     -e{toxinidir}[format]
 
-    py{27,35,py,py3}: green
+    py{27,35,py,py3},coverage: green
+    coverage: coverage
 
-    py{27,py,py3}: mock
+    py{27,py,py3},coverage: mock
 
     py{27,35}: lxml
     py{27,35,py}: sphinx
@@ -41,8 +42,6 @@ commands =
     coverage run --branch --source {toxinidir}/jsonschema [] {envbindir}/green jsonschema
     coverage report --show-missing
     coverage html
-deps =
-    coverage
 
 
 [testenv:docs]