situation with www/node6 and www/node

[Luca Pizzamiglio]

Hi,

node6 is the LTS, node is the current. From a stability point of view,
node6 is the choice, but node (7) is already widely used.
Probably, the best solution would be to provide the desired node
version via Mk/bsd.default-versions.mk and then all ports depends to
the common version (like perl, python, ...).

In the meanwhile, node can be the new default for yarn and the
conflict will be solved (and it will be coherent with npm)