Add back x86_64-darwin-16 to lockfile platforms
Add back x86_64-darwin-16 to lockfile platforms

This diff keeps happening now when bundling locally on my Mac; it was
somehow removed in
https://github.com/octobox/octobox/commit/db6c4c16b812bcc8a30af15f0796240c2f5cdc4f#diff-e79a60dc6b85309ae70a6ea8261eaf95

Bundled with Ruby 2.5.0 and Bundler 1.16.1

octobox/octobox,octobox/octobox,octobox/octobox,octobox/octobox