Fix the problem that statsmon node always exit

[quesalltime]

When using with netstats (for example: docker-compose scale bootstrap=1 miner=2 eth=3 statsmon=1) ,the statsmon node always exit.

Use "docker-compose logs" to check , the statsmon node shows error message as following:

statsmon | /eth-netstats/node_modules/primus/index.js:177 statsmon | const sandbox = Object.keys(global).reduce((acc, key) => { statsmon | ^ statsmon | statsmon | SyntaxError: missing ) after argument list statsmon | at exports.runInThisContext (vm.js:53:16) statsmon | at Module._compile (module.js:413:25) statsmon | at Object.Module._extensions..js (module.js:448:10) statsmon | at Module.load (module.js:355:32) statsmon | at Function.Module._load (module.js:310:12) statsmon | at Module.require (module.js:365:17) statsmon | at require (module.js:384:17) statsmon | at Object. (/eth-netstats/app.js:44:14) statsmon | at Module._compile (module.js:430:26) statsmon | at Object.Module._extensions..js (module.js:448:10) statsmon | statsmon | npm info eth-netstats@0.0.9 Failed to exec start script statsmon | npm ERR! Linux 3.13.0-107-generic statsmon | npm ERR! argv "/usr/local/bin/iojs" "/usr/local/bin/npm" "start" statsmon | npm ERR! node v3.3.0 statsmon | npm ERR! npm v2.13.3

It seems to be the same issue like this: https://github.com/Capgemini-AIE/ethereum-docker/issues/17

After modifying base image (replace "FROM iojs" with "FROM node:7") ,the statsmon node could work normally.