Fix install when use npm

[MrJmpl3]

The yarn command in install.js make a error when you don't use yarn or don't have yarn installed.

Maybe related: #35

[hawkeye64]

@MrJmpl3 You don't need a prompt. You can look for the existence of yarn.lock or package-lock.json to know if the project is yarn or npm based, respectively

[IlCallo]

Solved by #48