MAINT: update .travis.yml

* fix warnings in `.travis.yml` based on feedback from
https://config.travis-ci.com/explore

* attempting to restore Travis CI runs to our wheels
build matrix

.travis.yml

@@ -21,11 +21,11 @@ env:
 language: python
 # Default Python version is usually 2.7
 python: 3.5
-sudo: required
+os: linux
 dist: trusty
 services: docker
 
-matrix:
+jobs:
   exclude:
       # Exclude the default Python 3.5 build
       - python: 3.5