Update minimum Node version on README.md and package.json

[luixxiul]

Fixes #9797

Auditors:

Test Plan:

1. Update Node to >=7.9.0
2. npm install

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

[bsclifton]

Doc changes look great, npm install works great on macOS, Linux, and Windows (I wanted to check since the node-gyp version was bumped)

Thanks for catching this and updating 😄 👍