chore(travis): fix packagelock for node v8

CI was broken because the npm which ships with node v8 (and only that
version of npm, for some reason), would modify package-lock.json when we
ran `npm install`. This triggered our post-test hook which enforces docs
changes: the modified file was detected as a false-positive for
uncommitted docs changes.

This fixes the issue by committing those docs changes. This seems to be
compatible with other npm versions.

Fixes #893