Revert "Merge pull request #297 from airbrake/remove-js-notifier"

This reverts commit bb01b3b26a4b4214839b553d4afd6b1c4020d768, reversing
changes made to 85c611fb6e68aa21ceef2ca4941fdfc54af0ad69.
Add deprecation warning when using  .
This ensures we support what is expected until the next major release.
Bumping to version 3.2.1