Remove pytest-xdist in CI builds

It doesn't speed up tests that much and seems to conflict with coverage
in Python 3.7

setup.cfg

@@ -42,7 +42,6 @@ bash =
     argcomplete
 test =
     pytest-cov
-    pytest-xdist
     responses
 
 [bdist_wheel]

tox.ini

@@ -3,7 +3,7 @@ envlist = py27, py34, py35, py36, py37, flake8
 
 [testenv]
 commands =
-    {envbindir}/py.test --tb=short -n 4 \
+    {envbindir}/py.test --tb=short \
         --junitxml={toxworkdir}/junit-{envname}.xml \
         --cov=dosagelib --cov-config=tox.ini --cov-branch \
         --cov-report=term \