Upgrade node SDK support to requiring node LTS (8.9.0+)

now that node.js engine LTS has been updated from 6.9 to 8.9, we should move the SDK support to ^8.9.0 accordingly, and remove support for 6.9.

Although, it would be a good idea to keep the implementation to be node 6 compliant, until such a time when:

• a node 8-only feature is required, or
• using a node 8-only feature significantly improves the quality of the implementation

As a result, CI jobs running on node 6 should continue for now, along with node 8 CI jobs.