Use PyInstaller for freeze test env

cx_freeze doesn't seem to be very well supported in Python 3.5.

Using pyinstaller instead and rename environment to "freeze" which
is a more generic term for freezing python code into standalone
executables.

Fix #1769

_pytest/genscript.py

@@ -100,7 +100,8 @@ def pytest_namespace():
 def freeze_includes():
     """
     Returns a list of module names used by py.test that should be
-    included by cx_freeze.
+    included by cx_freeze/pyinstaller to generate a standalone
+    pytest executable.
     """
     result = list(_iter_all_modules(py))
     result += list(_iter_all_modules(_pytest))