Stop trying to use pytest-xdist when using pytest-cov

It works fine for me on my Mac, but it doesn't seem to work right on either TravisCI or AppVeyor.
It appears to calculate the code coverage incorrectly when using pytest-xdist.

Author: tleonhardt

tox.ini

@@ -12,11 +12,9 @@ deps =
   pyperclip
   pytest
   pytest-cov
-  pytest-xdist
   six
 commands =
-  py.test -v -n2 --cov=cmd2 --basetemp={envtmpdir} {posargs}
-  {envpython} examples/example.py --test examples/exampleSession.txt
+  py.test -v --cov=cmd2 --basetemp={envtmpdir} {posargs}
   codecov
 
 [testenv:py33]
@@ -57,11 +55,9 @@ deps =
   pyperclip
   pytest
   pytest-cov
-  pytest-xdist
   six
 commands =
-  py.test -v -n2 --cov=cmd2 --basetemp={envtmpdir} {posargs}
-  {envpython} examples/example.py --test examples/exampleSession.txt
+  py.test -v --cov=cmd2 --basetemp={envtmpdir} {posargs}
   codecov
 
 [testenv:py37]