test(ci): replace node v7.x.x with node v9.x.x
test(ci): replace node v7.x.x with node v9.x.x

We only want to test against the latest non-stable version. Also, greenkeeperio/greenkeeper-lockfile#98 is fixed so we can revert the switch to npm.

eventualbuddha/decaffeinate,alangpierce/decaffeinate,alangpierce/decaffeinate,eventualbuddha/decaffeinate,decaffeinate/decaffeinate,decaffeinate/decaffeinate,alangpierce/decaffeinate