Add to specify npm version

[asmsuechan]

context

grunt pre-build does not work on npm v0.5.3. So I edited docs.

What this below happens when I built Boostnote on npm v0.5.3.

[ Boostnote 18:23 ]% sudo grunt pre-build                                                                         [fix-app-name-and-icon-on-linux]
secret/auth_code.json is not found. CodeSigning is not available.
Running "pre-build" task

Running "compile" task
node_modules/.bin/webpack --config webpack-production.config.js
Hash: d37faa63ccf9b191659d
Version: webpack 1.15.0
Time: 30916ms
    Asset     Size  Chunks             Chunk Names
  main.js  2.11 MB       0  [emitted]  main
finder.js  1.68 MB       1  [emitted]  finder
    + 911 hidden modules


Running "pack:linux" (pack) task
/home/asmsuechan/src/asmsuechan/Boostnote/dist
Packaging app for platform linux x64 using electron v1.2.8
Error: Command failed: npm prune --production
npm ERR! May not delete: /tmp/electron-packager/linux-x64/Boostnote-linux-x64/resources/app/node_modules/.bin

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/asmsuechan/.npm/_logs/2017-08-18T09_24_14_807Z-debug.log

Warning: Command failed: npm prune --production
npm ERR! May not delete: /tmp/electron-packager/linux-x64/Boostnote-linux-x64/resources/app/node_modules/.bin

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/asmsuechan/.npm/_logs/2017-08-18T09_24_14_807Z-debug.log
 Use --force to continue.

Aborted due to warnings.

ref

npm/npm#17781
https://stackoverflow.com/questions/45229218/unable-to-build-with-electron-packager-within-electron-forge/45234407