Drop back to Hibernate 5.0.9.Final

The upgrade to Hibernate 5.2.0.Final has provide to be too
problematic to live with. It requires Java 8, is incompatible with
a number of other projects in the Hibernate ecosystem, and it's
unclear for how long it will be maintained. We'd previously used
Hibernate 5.1.0.Final but its maintenance is also unclear with
Hibernate 5.1.1.Final being more than 3 months overdue.

This commit drops back to Hibernate 5.0.9.Final. This has a few
advantages:

- It's Java 7 compatible
- It's had some time to mature and should be reasonably free of
  regressions for those moving from 4.3.x
- It's used in both Wildfly and JBoss EAP so there's a fair chance
  that it will continue to be maintained.

Closes gh-6198

spring-boot-devtools/pom.xml

@@ -51,7 +51,7 @@
 		</dependency>
 		<dependency>
 			<groupId>org.hibernate</groupId>
-			<artifactId>hibernate-core</artifactId>
+			<artifactId>hibernate-entitymanager</artifactId>
 			<optional>true</optional>
 		</dependency>
 		<dependency>