Add support for Node 5 and Node 6

[ghost]

If I install this plugin using Node 4, everything is ok. While if I install it with Node 5.9.1 or 6, I will receive the following errors:

npm WARN deprecated phantom@0.8.4: v1 is no longer maintained, please upgrade to v2.0+ as soon possible.
npm WARN deprecated win-spawn@2.0.0: use [cross-spawn](https://github.com/IndigoUnited/node-cross-spawn) or [cross-spawn-async](https://github.com/IndigoUnited/node-cross-spawn-async) instead.
npm WARN enoent ENOENT: no such file or directory, open '/Users/georgejor/projects/<repo>/node_modules/app/package.json'
npm ERR! Darwin 15.5.0
npm ERR! argv "/usr/local/Cellar/nvm/0.30.1/versions/node/v6.0.0/bin/node" "/usr/local/opt/nvm/versions/node/v6.0.0/bin/npm" "i" "--save" "gitbook-plugin-mermaid"
npm ERR! node v6.0.0
npm ERR! npm  v3.8.6
npm ERR! path /Users/georgejor/projects/<repo>/node_modules/.staging/sockjs-client-48d6b838
npm ERR! code ENOENT
npm ERR! errno -2
npm ERR! syscall rename

npm ERR! enoent ENOENT: no such file or directory, rename '/Users/georgejor/projects/<repo>/node_modules/.staging/sockjs-client-48d6b838' -> '/Users/georgejor/projects/<repo>/node_modules/shoe/node_modules/sockjs-client'
npm ERR! enoent ENOENT: no such file or directory, rename '/Users/georgejor/projects/<repo>/node_modules/.staging/sockjs-client-48d6b838' -> '/Users/georgejor/projects/<repo>/node_modules/shoe/node_modules/sockjs-client'
npm ERR! enoent This is most likely not a problem with npm itself
npm ERR! enoent and is related to npm not being able to find a file.
npm ERR! enoent 

Please help add support for such issue, thanks