Enable 3.7 ci

Author: Yury Selivanov

.travis.yml

@@ -56,6 +56,13 @@ matrix:
           python: "3.6"
           env: BUILD=tests
 
+        - os: linux
+          dist: xenial
+          sudo: required
+          language: python
+          python: "3.7"
+          env: BUILD=tests
+
         - os: linux
           dist: trusty
           branches: {only: [releases]}