Declare Python 3.6 support (#93)

Add Python 3.6 to the Trove classifiers in `setup.py`.
diff --git a/setup.py b/setup.py
index dc2ce39..81302bf 100755
--- a/setup.py
+++ b/setup.py
@@ -40,6 +40,7 @@
               'Programming Language :: Python :: 3.3',
               'Programming Language :: Python :: 3.4',
               'Programming Language :: Python :: 3.5',
+              'Programming Language :: Python :: 3.6',
               'Topic :: Security :: Cryptography',
           ],
           install_requires=[