Declare support for Python 3.6

Per the [changelog](https://github.com/pypa/virtualenv/blob/6d8a4ed81f40223a3f9ea1ba5d11be500d9f667a/docs/changes.rst)
notes for `15.1.0`, Python 3.6 is supported. This change adds 3.6 to the
trove classifiers and drops the now-unsupported 3.3.

.travis.yml

@@ -15,7 +15,7 @@ matrix:
       env: TOXENV=python3.6
     - python: pypy
       env: TOXENV=pypy
-    - python: 3.5
+    - python: 3.6
       env: TOXENV=crosspython3
     - python: 2.7
       env: TOXENV=crosspython2

setup.py

@@ -106,9 +106,9 @@ def get_version():
         'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
+        'Programming Language :: Python :: 3.6',
     ],
     keywords='setuptools deployment installation distutils',
     author='Ian Bicking',