use matrix for different python version

.travis.yml

@@ -1,14 +1,14 @@
 sudo: required
 
-dist: xenial
 language: python
-python:
-- 2.7
-- 3.3
-- 3.4
-- 3.5
-- 3.6
-- 3.7
+matrix:
+  include:
+    - python: 2.7
+    - python: 3.4
+    - python: 3.5
+    - python: 3.6
+    - python: 3.7
+      dist: xenial
 
 before_script:
   - sh ./travis/init.sh