Update Node instructions to v10.x
Update Node instructions to v10.x

The required Node version was updated from v6.x to v10.x.

This commit updates the docs to reflect that.

Ref: learningequality/kolibri#4524
Signed-off-by: Nick Cannariato <38f26356ca1352b2885e5ed2a471115e7b1d2b45@users.noreply.github.com>

indirectlylit/kolibri,indirectlylit/kolibri,lyw07/kolibri,learningequality/kolibri,indirectlylit/kolibri,learningequality/kolibri,mrpau/kolibri,mrpau/kolibri,lyw07/kolibri,lyw07/kolibri,mrpau/kolibri,learningequality/kolibri,mrpau/kolibri,learningequality/kolibri,lyw07/kolibri,indirectlylit/kolibri