fix build in mqttjs

[scarry1992]

replace arrow function with usual declaration.

[scarry1992]

@mcollina, can you release that fix asap, that`s cracks install of mqtt.js

[mcollina]

Which fix?

[scarry1992]

@mcollina, mqttjs failed building with that version of mqtt-packet, I was wrong with arrow function

[scarry1992]

Parse error at 0:9030,65
npm ERR! prepareGitDep result += currentValue.reduce((currentLength, value) => {
npm ERR! prepareGitDep ^
npm ERR! prepareGitDep ERROR: Invalid assignment
npm ERR! prepareGitDep at JS_Parse_Error.get (eval at (/home/sebu/.npm/_cacache/tmp/git-clone-ea9541ac/node_modules/uglify-js/tools/node.js:20:1), :71:23)
npm ERR! prepareGitDep at fatal (/home/sebu/.npm/_cacache/tmp/git-clone-ea9541ac/node_modules/uglify-js/bin/uglifyjs:296:53)
npm ERR! prepareGitDep at run (/home/sebu/.npm/_cacache/tmp/git-clone-ea9541ac/node_modules/uglify-js/bin/uglifyjs:240:9)
npm ERR! prepareGitDep at ReadStream. (/home/sebu/.npm/_cacache/tmp/git-clone-ea9541ac/node_modules/uglify-js/bin/uglifyjs:173:9)
npm ERR! prepareGitDep at ReadStream.emit (events.js:202:15)
npm ERR! prepareGitDep at endReadableNT (_stream_readable.js:1129:12)
npm ERR! prepareGitDep at processTicksAndRejections (internal/process/next_tick.js:76:17)

[scarry1992]

@mcollina, need release in npm with fix version 6.1.1