Enable branch coverage (and newer tox features).

tox.ini

@@ -3,17 +3,22 @@ envlist = py27, py34, py35, py36, flake8
 
 [testenv]
 commands =
-    {envbindir}/py.test --cov=dosagelib --tb=short -n4 \
-        --cov-report=xml:{toxworkdir}/cov-{envname}.xml --cov-report=term \
-        --junitxml={toxworkdir}/junit-{envname}.xml {posargs}
+    {envbindir}/py.test --tb=short -n auto \
+        --junitxml={toxworkdir}/junit-{envname}.xml \
+        --cov=dosagelib --cov-config=tox.ini --cov-branch \
+        --cov-report=term \
+        --cov-report=xml:{toxworkdir}/cov-{envname}.xml \
+        {posargs}
 
 
 passenv = CI TRAVIS TRAVIS_*
-# Also install optional dependencies for tests.
 deps =
-    .[css,dimensions,test]
     pytest-travis-fold
-    Pillow
+# Also install extra dependencies for tests.
+extras =
+    css
+    dimensions
+    test
 
 [testenv:flake8]
 commands =