tox: Test if coverage data is readable if not using pytest-xdist

tox.ini

@@ -3,19 +3,17 @@ envlist = py27, py27-tornado23, py27-tornado31, docs, flake8
 
 [testenv]
 sitepackages = true
-commands =
-    py.test \
-        --basetemp={envtmpdir} \
-        --cov=mopidy --cov-report=term-missing \
-        -n 4 \
-        {posargs}
 deps =
     mock
     pytest
     pytest-capturelog
     pytest-cov
-    pytest-xdist
     responses
+commands =
+    py.test \
+        --basetemp={envtmpdir} \
+        --cov=mopidy --cov-report=term-missing \
+        {posargs}
 
 [testenv:py27-tornado23]
 commands = py.test tests/http