Use `npm ci` to install dependencies
Use `npm ci` to install dependencies

By default Travis runs `npm install`. This can end up modifying
`package-lock.json` if Travis has a different version of npm than
whoever committed the last changes to the lockfile.

`npm ci` should avoid this problem by installing _only_ from the
lockfile. This command also promises faster installation. Since this
command was introduced in npm v5.7, we need to first install a newer
version of npm.

 - Change the cache directory as per the `npm ci` docs.

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