should change to a stable NodeJS version

[OriHoch]

@noam-almog wrote:

as for node crashing for no good reason, we are using node 7.4 which is by definition unstable, we should either consider downgrading to 6.10.2 or upgrade to latest unstable release (7.9)

[roy-zahor]

We are now using node features that are not available in 7.4.0. So, in .nvmrc file I have updated to 7.9.0.
Production server needs to be updated as well, see #371

[noam-almog]

@roy-zahor 7.10.0 is out, better to use it, no ?

[roy-zahor]

@noam-almog I guess you are right

[roy-zahor]

The issue is closed, but I comment to have a documentation:
As discussed yesterday, we will keep 7.9.0 at least for the next month up until the event.