Fixes a security vulnerability by updating jpeg-js dependency.

Drops support for Node <8. Technically a breaking change, but as major consumers of speedline (lighthouse, browsertime) both require Node 10 and yarn itself has long dropped support for our current Node 5 without a breaking change. We decided to treat as a bug fix patch release.

Fixes:

• Exclude screenshots that occurred before the timeOrigin

Fixes:

• perceptual-speed-index: use proper SSIM options to avoid incorrect results (#42)

Code Health:

• Add yarn.lock to manage dependencies