Use Electron instead of PhantomJS

[lastmjs]

Why is this project using PhantomJS? Electron is more up-to-date and is even coming with true headless mode soon. Seems like a better choice.

[schickling]

Hi @lastmjs! We're currently rewriting prep to use a headless version of Chrome capable of running on serverless functions. Stay tuned!

[baebb]

@schickling on serverless functions??? hyper-curious now

[stereobooster]

it is already based on chromeless. This can be closed

UPD: I was confused by note in readme. Actually it uses nightmare, which uses electron. Documentation need to be fixed :/