Update base node to version 8 (or even 10, as it's the current LTS)

[elboletaire]

Version 6 of node is no longer an Active LTS version. The current LTS versions are 8 and 10. BTW, trying to update to version 10 can be almost impossible due to the dependencies (I've not tried with OC, but with custom projects that are currently using version 8, trying to update to version 10 its impossible: many dependencies are not properly updated).

Right now, if you clone the code and use node 6, there's some dependency which already crashes due to lack of compatibility with node 6 (I think it's related to hasky, see #961):

undefined requires at least version 8.6.0 of Node, please upgrade

Version 6 end date is April 2019.

I've already tried to locally update nodejs to version 8 and, apparently, everything works as expected. If you can confirm me that "everything" is tested (720 tests properly passed) and that we can trust them, I'll be glad to create a PR with the change.

There are some deprecation warnings but they also appear under version 6, so I guess that's expected.