The version set for Pac4j was previously ineffective because it was set
by spring-security-pac4j first.

The Spring Platform BOM has reached EOL, so we need to replace it for
future updates. This should not change any dependency versions.

https://spring.io/blog/2018/04/09/spring-io-platform-end-of-life-announcement

See GH-65.

Google shuts down its Google+ Sign-In.

Fixes regression introduced by commit
c88cce27.