Refactor tox and ci scripts

Make sure a plain "tox" command works in all platforms without failures,
which facilitates making releases using a "devpi upload/test" workflow

* Fix small flakes failures
* Limit py35 testing to pytest>=2.7, as pytest<2.7 does not work on py35
* pexpect environments only work on Linux platforms and should be skipped on Windows
* Simplify AppVeyor script by running all tox environments: no need to have a build matrix
  because AppVeyor does not execute builds in parallel
* Add all environments to travis.yml, obtained from "tox --listenvs"

xdist/__init__.py

@@ -1,2 +1,3 @@
-__all__ = ['__version__']
 from xdist._version import version as __version__
+
+__all__ = ['__version__']