Added support for building with node 6.
Added support for building with node 6.

node-mapnik version 3.5.13 doesn't build with node v6
and pre-built binaries used by node-pre-gyp are built
using a newer C++ compiler and won't run in our production
environment. However, it's possible to build node-mapnik
from scratch in the docker environment making it possible
to deploy kartotherian using our traditional deployment
method.

To make this work the npm-shrinkwrap.json should be removed from
the deploy repo and https://github.com/wikimedia/service-runner/pull/140
should be morged.

Change-Id: I5ec951b00fd4b80ac8041afc18257aeec8496512

nyurik/kartotherian,nyurik/kartotherian,kartotherian/kartotherian,nyurik/kartotherian,nyurik/kartotherian,kartotherian/kartotherian,wikimedia/maps-kartotherian,kartotherian/kartotherian,wikimedia/maps-kartotherian,kartotherian/kartotherian