fix(ci): Update Travis CI script so greenkeeper works with the lockfile

[ChristianMurphy]

1. Detect whether the current CI build is caused by Greenkeeper
2. Update the lockfile with the latest version of the updated dependency using the package manager’s built in mechanism
3. Push a commit with the updated lockfile back to the Greenkeeper branch

reference: https://github.com/greenkeeperio/greenkeeper-lockfile#how-does-it-work

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