Add support for Python 3.6

Also convert to zope.testrunner in tox.ini and .travis.yml because of
the namespace package issue:

```
$ tox -e py36
GLOB sdist-make: //zope.proxy/setup.py
py36 create: //zope.proxy/.tox/py36
py36 installdeps: .[test]
py36 inst: //zope.proxy/.tox/dist/zope.proxy-4.2.1.dev0.zip
py36 installed: appdirs==1.4.3,packaging==16.8,pyparsing==2.2.0,six==1.10.0,zope.component==4.3.0,zope.event==4.2.0,zope.i18nmessageid==4.0.3,zope.interface==4.4.0,zope.location==4.0.3,zope.proxy==4.2.1.dev0,zope.schema==4.4.2,zope.security==4.0.3
py36 runtests: PYTHONHASHSEED='770288004'
py36 runtests: commands[0] | python setup.py -q test -q
warning: no previously-included files matching '*.dll' found anywhere in distribution
warning: no previously-included files matching '*.pyo' found anywhere in distribution
E
======================================================================
ERROR: proxy (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: proxy
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/loader.py", line 153, in loadTestsFromName
    module = __import__(module_name)
  File "//zope.proxy/src/zope/proxy/__init__.py", line 21, in <module>
    from zope.interface import moduleProvides
ModuleNotFoundError: No module named 'zope.interface'
```

Also enable travis pip cache.

.travis.yml

@@ -5,10 +5,17 @@ python:
     - 3.3
     - 3.4
     - 3.5
+    - 3.6
     - pypy
 install:
+    - pip install -U pip
+    - pip install -U setuptools
+    - pip install -U zope.testrunner
     - pip install -e .[test]
 script:
-    - python setup.py test -q
+    - zope-testrunner --test-path=src
 notifications:
     email: false
+cache: pip
+before_cache:
+    - rm -f $HOME/.cache/pip/log/debug.log

CHANGES.rst

@@ -7,6 +7,8 @@ Changes
 - Make the pure-Python implementation of ``sameProxiedObjects`` handle
   ``zope.security`` proxies. See `issue 15 <https://github.com/zopefoundation/zope.proxy/issues/15>`_.
 
+- Add support for Python 3.6.
+
 4.2.0 (2016-05-05)
 ------------------
 

setup.py

@@ -71,6 +71,7 @@ def read(*rnames):
           'Programming Language :: Python :: 3.3',
           'Programming Language :: Python :: 3.4',
           'Programming Language :: Python :: 3.5',
+          'Programming Language :: Python :: 3.6',
           'Programming Language :: Python :: Implementation :: CPython',
           'Programming Language :: Python :: Implementation :: PyPy',
           "Framework :: Zope3",

tox.ini

@@ -3,13 +3,14 @@ envlist =
 # Jython support pending 2.7 support, due 2012-07-15 or so.  See:
 # http://fwierzbicki.blogspot.com/2012/03/adconion-to-fund-jython-27.html
 #   py27,jython,pypy,coverage
-    py27,py27-pure,py33,py33-pure,py34,py35,pypy,coverage,docs
+    py27,py27-pure,py33,py33-pure,py34,py35,py36,pypy,coverage,docs
 
 [testenv]
 commands =
-    python setup.py -q test -q
+    zope-testrunner --test-path=src
 deps =
      .[test]
+     zope.testrunner
 
 [testenv:py27-pure]
 basepython =