chore(deps): bump ssri and npm

Removes [ssri](https://github.com/npm/ssri). It's no longer used after updating ancestor dependency [npm](https://github.com/npm/cli). These dependencies need to be updated together.


Removes `ssri`

Updates `npm` from 6.14.11 to 6.14.18
- [Release notes](https://github.com/npm/cli/releases)
- [Changelog](https://github.com/npm/cli/blob/v6.14.18/CHANGELOG.md)
- [Commits](npm/cli@v6.14.11...v6.14.18)

---
updated-dependencies:
- dependency-name: ssri
  dependency-type: indirect
- dependency-name: npm
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>