Ignore package-lock.json temporarily

[luixxiul]

Let's revert it after travis-ci/travis-ci#7936 is fixed. See: #9691 (comment)

Addresses #8283

Test Plan:

1. rm -rf node_modules
2. npm install
3. git status
4. make sure package-lock.json is no longer listed

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bbondy]

We should instead be committing a package-lock.json and not ignoring it.

[luixxiul]

I see!