Also update to nan@2.14.0 to prevent build errors with newer Node.js
versions. This is similar to bnoordhuis/node-heapdump@8b1896f6919 and
not really a node-iconv bug.

Fixes: https://github.com/bnoordhuis/node-iconv/issues/202
Refs: https://github.com/bnoordhuis/node-heapdump/issues/129