Attempt to speedup unit tests

Now that we have a lot of unit tests and are running code coverage analysis,
our unit tests take a long time to run.

Changes include ...

tox:
- Install pytest-xdist to parallelize unit tests
- Only run code coverage analysis on Python 2.7 and 3.6
- Don't run examples/example.py anymore, just run the unit tests

TravisCI:
- Stop building on Python 3.7-dev since that won't be released for some time

AppVeyor:
- Stop building on Python 3.4 since that isn't as important and is covered by Travis

Author: tleonhardt

.appveyor.yml

@@ -4,4 +4,4 @@ install:
 build: off
 
 test_script:
-  - python -m tox -e py27,py34,py35,py36
+  - python -m tox -e py27,py35,py36

.travis.yml

@@ -19,9 +19,9 @@ matrix:
         - os: linux
           python: 3.6
           env: TOXENV=py36
-        - os: linux
-          python: 3.7-dev
-          env: TOXENV=py37
+#        - os: linux
+#          python: 3.7-dev
+#          env: TOXENV=py37
 #        # Warning: Don't try to use code coverage analysis with pypy as it is insanely slow
 #        - os: linux
 #          python: pypy

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py34,py35,py36,jython,pypy
+envlist = py27,py33,py34,py35,py36,py37,pypy
 
 [testenv]
 passenv = CI TRAVIS TRAVIS_* APPVEYOR*
@@ -10,9 +10,30 @@ deps =
   pyperclip
   pytest
   pytest-cov
+  pytest-xdist
   six
-commands=
-  py.test -v --cov=cmd2 --basetemp={envtmpdir} {posargs}
-  {envpython} examples/example.py --test examples/exampleSession.txt
+
+[testenv:py27]
+commands =
+  codecov erase
+  py.test -v -n2 --cov=cmd2 --basetemp={envtmpdir} {posargs}
   codecov
 
+[testenv:py33]
+commands = py.test -v -n2
+
+[testenv:py34]
+commands = py.test -v -n2
+
+[testenv:py35]
+commands = py.test -v -n2
+
+[testenv:py36]
+commands =
+  codecov erase
+  py.test -v -n2 --cov=cmd2 --basetemp={envtmpdir} {posargs}
+  codecov
+
+[testenv:py37]
+commands = py.test -v -n2
+