Upgrade to Hibernate 5.1, whilst still retaining compatibility with
Hibernate 4.3. This commit introduces the following changes:

* Add SpringPhysicalNamingStrategy to provides lowercase/underscore
  table names support. This should be equivalent to the previous
  SpringNamingStrategy that was used with Hibernate 4. No
  ImplicitNamingStrategy is provided since the Hibernate 5 defaults
  appear to be roughly equivalent to the conventions used in Spring
  Boot 1.3
  spring.jpa.hibernate.naming.

* Migrate  `spring.jpa.hibernate.naming-strategy` to
  `spring.jpa.hibernate.naming.strategy` and provide additional
  properties for physical and implicit.

* Add `spring.jpa.hibernate.use-new-id-generator-mappings` property and
  default to `false` when on Hibernate 5 to retain back compatibility.

See gh-2763