Avoid nose, use coverage run directly

Because the nose-xcover plugin does something that breaks coverage
reporting in a way that makes it output the coverage of the entire
Python standar library in addition to the code we care about.

Also, this way tox.ini and .travis.yml use the same test command, which
is nice.

tox.ini

@@ -17,8 +17,8 @@ commands =
 #   version, before running nosetests.
     pip uninstall -y ZConfig
     pip install -e .[test]
-    nosetests --with-xunit --with-xcoverage
+    coverage run setup.py -q test -q
+    coverage report -m
 deps =
-    nose
     coverage
-    nosexcover
+    {[testenv]deps}