Revert "Merge branch 'refs/heads/notifiable_0_18_0' into dev"

This reverts commit b28f906e16ac770ab7b24744e2abc39da8e488f3, reversing
changes made to d2f8e45da283988aed28218a04e3dbb460f1a266.
Bump notifiable-rails
Remove unique index from token
Bump version