Use group: travis_latest instead of dist: trusty to pick up the latest
Travis CI image changes. The new default is group: travis_lts which is
equivalent to the now-deprecated dist: trusty.

More info in this blog post:
https://blog.travis-ci.com/2017-12-01-new-update-schedule-for-linux-build-images

Fixes #811.

Thank you for contributing to JanusGraph!

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there an issue associated with this PR? Is it referenced in the commit message?
• Does your PR body contain #xyz where xyz is the issue number you are trying to resolve?
• Has your PR been rebased against the latest commit within the target branch (typically master)?
• Is your initial contribution a single, squashed commit?

For code changes:

• [] Have you written and/or updated unit tests to verify your changes?
• [] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [] If applicable, have you updated the LICENSE.txt file, including the main LICENSE.txt file in the root of this repository?
• [] If applicable, have you updated the NOTICE.txt file, including the main NOTICE.txt file found in the root of this repository?

For documentation related changes:

• [] Have you ensured that format looks appropriate for the output in which it is rendered?
• [] If this PR is a documentation-only change, have you added a [skip ci]
  tag to the first line of your commit message to avoid spending CPU cycles in
  Travis CI when no code, tests, or build configuration are modified?

Note:

Please ensure that once the PR is submitted, you check Travis CI for build issues and submit an update to your PR as soon as possible.