chore: add Node.js 6, 8 and 10, remove Node.js 4 (#54)
chore: add Node.js 6, 8 and 10, remove Node.js 4 (#54)

* chore: add Node.js 6, 8 and 10, remove Node.js 4

* chore: only test Node.js 8 and 10

imgix/luminous,imgix/luminous