fix: require django-storages and boto for coverage

otherwise the coverage env is missing these dependencies

tox.ini

@@ -3,13 +3,19 @@ envlist=
     py27-django{18,19,110,111},
     py36-django{18,19,110,111},
     py36-django{20},
+    py36-django{21},
+    py36-django{22},
     # py36-django-dev,
     coverage,
 
-[testenv]
+[base]
 deps =
     django-storages
     boto3
+
+[testenv]
+deps =
+    {[base]deps}
     django18: Django >= 1.8,<1.9
     django19: Django >= 1.9,<1.10
     django110: Django >= 1.10,<1.11
@@ -24,6 +30,7 @@ basepython=python3.6
 deps=
     django==1.11
     coverage
+    {[base]deps}
 commands=
     coverage erase
     coverage run --rcfile=.coveragerc runtests.py