Since v8 is the LTS version of NodeJS, and we use Nodejs only for development, test and build, I think we should drop the support of v6 and move entirely to v8.
I am not sure about the build process, but travis run tests against 8, 6 and 6.10:
- '6.10' # current EPEL 7
- '6' # current LTS
- '8' # current stable