Add group field to .travis.yml (#289)
Add group field to .travis.yml (#289)

Travis CI recently added `group` filed to specify which image to be used. `group: travis_lts` (default) is a stable image while `group: travis_latest` is an image that is frequently being updated with new packages.

Related links:
* https://blog.travis-ci.com/2017-12-12-new-trusty-images-q4-launch
* https://blog.travis-ci.com/2017-12-01-new-update-schedule-for-linux-build-images
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