Change module system of the main file

[thiamsantos]

I'm trying to use rollup as build tool for redux-logger following the example of what react did. But I'm encountering a problem: rollup doesn't work well with umd builds(because it works with es2015 modules) and the only file exported by this project is an umd. So, I would like to send a PR to solve this problem.

For more info about why to use rollup you can read Webpack and Rollup: the same but different that explain the main reasons why rollup is better for redux-logger.

Thanks for your time 😉

[flitbit]

Completed with v0.3.5 — thanks @thiamsantos !