Update README.md

Version upgraded to 4.0.1

Update README.md

Added notes for Version 3.2.0

Set higher defaulf versions for GRADLE_TOOLS_VERSION and GOOGLE_SERVI…

…CES_VERSION

Merge pull request andrehtissot#6 from andrehtissot/3.0.0-beta

3.0.0-beta small tweaks

Added mavenCentral() as gradle repository

Avoided remoteMessage.getNotification() null