have Travis test using Python 3.6 instead of 3.7

Attempting to use Python 3.7 showed this error:

3.7 is not installed; attempting download
Downloading archive: https://s3.amazonaws.com/travis-python-archives/binaries/ubuntu/14.04/x86_64/python-3.7.tar.bz2
$ curl -sSf -o python-3.7.tar.bz2 ${archive_url}
curl: (22) The requested URL returned error: 403 Forbidden
Unable to download 3.7 archive. The archive may not exist. Please consider a different version.

.travis.yml

@@ -1,6 +1,6 @@
 language: python
 python:
   - "2.7"
-  - "3.7"
+  - "3.6"
 script:
   - python test.py