travis: fix build config validation problems

- https://docs.travis-ci.com/user/reference/overview/#deprecated-virtualization-environments

.travis.yml

@@ -1,11 +1,12 @@
+---
+os: linux
 language: python
-sudo: false
 cache: pip
 
 git:
   submodules: true
 
-matrix:
+jobs:
   include:
   - python: "2.7"
     env: TOXENV=py27