Minimal nodejs should be checked

[ofaurax]

• [X] I have searched open and closed issues for duplicates

---

Bug Description

I'm unable to build Signal-Desktop on Mageia 6, because nodejs shipped with it is v6.10.3.

It seems that the "async" functions are not supported before nodejs 7.2.

However, I would expect the build to fail at the very beginning, not after downloading lots of npm modules.

Steps to Reproduce

1. Setup Mageia6 with nodejs 6.10.3
2. npm install

Actual Result: Build fail after download of modules

~/git/Signal-Desktop/node_modules/read-config-file/out/main.js:72
async function readConfig(configFile) {
      ^^^^^^^^
SyntaxError: Unexpected token function
    at createScript (vm.js:56:10)

Expected Result: As nodejs 7.2+ is required, build should fail at build start

Screenshots

Platform Info

Signal Version: git describe = 1.9.0-beta.1-596-gb816638d

Operating System: Mageia 6

Linked Device Version: ?

Link to Debug Log

npm ERR! Linux 4.14.89-desktop-1.mga6
npm ERR! argv "/usr/bin/node" "/usr/bin/npm" "install"
npm ERR! node v6.10.3
npm ERR! npm  v3.10.10
npm ERR! code ELIFECYCLE
npm ERR! signal-desktop@1.19.0-beta.4 postinstall: `electron-builder install-app-deps && rimraf node_modules/dtrace-provider`
npm ERR! Exit status 1

[scottnonnenberg-signal]

package.json specifies it here: https://github.com/signalapp/Signal-Desktop/blob/development/package.json#L149-L151

We use yarn in this project, which warns you on yarn install if your version of node.js is incorrect.