Add npm ci check

[ajlai]

npm 5.8.0 has a new command npm-ci which functions like npm install but also checks that the package-lock.json is up to date with the package.json file.

This PR changes the travis config to use npm ci.

#126

[ajlai]

For an example of how a travis failure looks when package-lock.json hasn't been updated, see https://travis-ci.org/ajlai/clasp/jobs/367926019

[coveralls]

Pull Request Test Coverage Report for Build 85

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 10.56%

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Totals
Change from base Build 83:	0.0%
Covered Lines:	93
Relevant Lines:	658

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💛 - Coveralls

[grant]

I appreciate the PR, but I don't think this PR speeds up the build.
https://travis-ci.org/google/clasp/builds/367971284

I'd remove osx / linux if the goal is to speed up Travis.

[ajlai]

My original goal wasn't to speed up the build (although that would have been nice). It's just to make sure package-lock.json gets updated and checked in when the package.json does.

[grant]

• Can you create an issue before creating a PR? The PR description is unclear what this is trying to solve.
• Can you add a better comment as to why we use ci?
  • "keep the npm cache around to speed up installs"
    • Seems like this PR is trying to speed up the install. ci seems to do something different (more stable install)
    • Change this comment to something more descriptive.
  • You can also link this in a comment: https://docs.npmjs.com/cli/ci

LGTM after those changes.

[grant]

Thanks! Looks good.