Declare support for Python 3.7 (#515)

I believe it would be nice to see support for
modern and upcoming versions of Python.

.travis.yml

+dist: bionic
 language: python
 python:
-- 3.6
-os: linux
+  - "3.6"
+  - "3.7"
+  - "3.8-dev"
+  - "nightly"
+matrix:
+  allow_failures:
+    - python: "3.8-dev"
+    - python: "nightly"
 install:
 - pip install -r requirements.txt
 script:

setup.py

@@ -55,6 +55,7 @@ setup(
 		'License :: OSI Approved :: MIT License',
 		'Programming Language :: Python',
 		'Programming Language :: Python :: 3',
-		'Programming Language :: Python :: 3.6'
+		'Programming Language :: Python :: 3.6',
+		'Programming Language :: Python :: 3.7',
 		],
 )