Update package for usage with rollup >= 1

[vladshcherbin]

Hey,

I'm using rollup 1.2.1 and while the package works, it gives a note:

'rollup-plugin-notify' will not work. Rollup 0.60.0 or higher is required

I see a check for minor version (<60) and in version 1.2.* it's equal to 2. Would be nice to have an update for current rollup version.

Also, success option is missing in the docs.

[MikeKovarik]

fixed with #4

[vladshcherbin]

@MikeKovarik thanks 😉