travis: fix build config validation problems

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

.travis.yml

@@ -1,24 +1,25 @@
+---
+os: linux
 language: python
-sudo: false
 cache: pip
 
 git:
   submodules: true
 
-matrix:
+jobs:
   include:
-    - python: "2.7"
-      env: TOXENV=py27
-    - python: "3.5"
-      env: TOXENV=py35
-    - python: "2.7"
-      env: TOXENV=py27
+  - python: "2.7"
+    env: TOXENV=py27
+  - python: "3.5"
+    env: TOXENV=py35
+  - python: "2.7"
+    env: TOXENV=py27
 
 install:
-  - pip install --ignore-installed --upgrade setuptools pip tox coveralls
+- pip install --ignore-installed --upgrade setuptools pip tox coveralls
 
 script:
-  - cd library
-  - tox -vv
+- cd library
+- tox -vv
 
 after_success: if [ "$TOXENV" == "py35" ]; then coveralls; fi