Note Python 3.7 support (#95)

Update the repo to list Python 3.7 support alongside other Python
versions that are supported.

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,6 +4,6 @@ Before you submit a pull request, check that it meets these guidelines:
 2. If the pull request adds functionality, the docs should be updated. Put
    your new functionality into a function with a docstring, and add the
    feature to the list in README.rst.
-3. The pull request should work for Python 2.7, 3.5, and 3.6. Check
+3. The pull request should work for Python 2.7, 3.5, 3.6, and 3.7. Check
    https://travis-ci.org/dask/dask-image/pull_requests
    and make sure that the tests pass for all supported Python versions.

setup.py

@@ -73,6 +73,7 @@ def run_tests(self):
         "Programming Language :: Python :: 3",
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.7",
     ],
     tests_require=test_requirements
 )