upgrade node-abi and prebuild for newest v4 with correct Electron ABI

[shiftkey]

Closes #133

node-abi now has support for the correct v4 Electron ABI electron/node-abi@9e4c1a6

This PR updates us to use that package and adds 4.0.4 as a target - after running npm run prebuild-electron locally I got an additional package with then new v69 ABI, ready to go:

$ ls prebuilds                                                                                                                            
keytar-v4.3.1-electron-v53-darwin-x64.tar.gz	keytar-v4.3.1-electron-v57-darwin-x64.tar.gz	keytar-v4.3.1-electron-v69-darwin-x64.tar.gz
keytar-v4.3.1-electron-v54-darwin-x64.tar.gz	keytar-v4.3.1-electron-v64-darwin-x64.tar.gz

[daviwil]

Looks good to me!