Switch to explicit matrix build

.travis.yml

@@ -1,11 +1,15 @@
 sudo: false
-dist: xenial
 language: python
-python:
+install: pip3 install tox-travis
-  - "3.4"
-  - "3.5"
-  - "3.6"
-  - "3.7"
-install: pip install tox-travis
 script: tox
-
+matrix:
+        include:
+                - os: "linux"
+                  dist: "xenial"
+                  python: "3.5"
+                - os: "linux"
+                  dist: "xenial"
+                  python: "3.6"
+                - os: "linux"
+                  dist: "xenial"
+                  python: "3.7"