Add invariant package as dependency instead of telling user to install fbjs

[Looooong]

Installing fbjs just to use the invariant method is somewhat overkill. Can we use the package invariant as a dependency of this library instead?

[Dallas62]

Hi, I'm also wondering why using fbjs and invariant, it's only used on time in the project and it's seems overkill to throw an Error. https://github.com/wonday/react-native-pdf/blob/ab11418713a5a5722b65688b2c7f4d2c01a0bae1/PdfManager.js Regards