Fixed README.md with libusb dev headers + correct nodejs version

[Someguy123]

Possibly fixes issue #313 and fixes issue #290

I had issues trying to run npm install on Ubuntu 18.04 Server, and after some trial and error I managed to get it working with the following workarounds:

• You MUST use an older NodeJS version such as v6.17.1 (last LTS for v6), as the current LTS pf 12.13.0 does NOT work. (this is why NVM exists)
  • nvm install v6.17.1
• You MUST install node-gyp before running npm install, otherwise it will fail to compile some of the dependencies and the install will error.
  • npm install node-gyp
• At least on Debian/Ubuntu, you MUST install libusb-1.0.0-dev or it will error saying it can't find libusb.h
  • apt install -y libusb-1.0-0-dev

I've updated the README.md and the readme in dapp/README.md to make these installation requirements clear, and save others having to figure out why it's broken.

[Someguy123]

Closed this PR as I made a new one with a branch privex-fix-readme (this PR tracks master which would include any other commits we make...)

New PR: https://github.com/gnosis/MultiSigWallet/pull/322