tox: Remove pytest-xdist

This seems to fix the Travis build that recently started getting an
ImportError on `gobject`.

tox.ini

@@ -7,13 +7,11 @@ commands =
     pytest \
         --basetemp={envtmpdir} \
         --cov=mopidy --cov-report=term-missing \
-        -n 4 \
         {posargs}
 deps =
     mock
-    pytest>=3.3.0
+    pytest
     pytest-cov
-    pytest-xdist
     responses
 
 [testenv:docs]