Support Spring 5 and Spring Boot 2.x

Hi @jirutka, 

I know that this project is not in your top priority list but would be really nice to support or completely move to spring 5.x since supporting all versions is real pain. Existing users can continue to use old versions. 

I would use Spring Boot Parent instead of Spring Io Bom by the way. It would make things less easier. What do you think?