Add engine to package.json such that only node LTS can be used

[TimDaub]

On node 6:

etherscan-uploader -V
/usr/local/lib/node_modules/etherscan-uploader/index.js:26
  )
  ^
SyntaxError: Unexpected token )
    at createScript (vm.js:56:10)
    at Object.runInThisContext (vm.js:97:10)
    at Module._compile (module.js:542:28)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)
    at run (bootstrap_node.js:393:7)
    at startup (bootstrap_node.js:150:9)

Maybe also fork https://github.com/poanetwork/solidity-flattener and change engine in package.json