Avoid exception from dropping non-existent constraint in Data REST sample

Hibernate 5.1 logs an ugly but benign exception when using an in-memory
HSQL database when it tries to drop a non-existent constraint.

This commit changes the Spring Data REST sample to use H2 rather than
HSQL. This prevents the unwanted exception from occurring as Hibernate's
H2 dialect returns false from dropConstraints.

spring-boot-samples/spring-boot-sample-data-rest/pom.xml

@@ -28,8 +28,8 @@
 			<artifactId>spring-boot-starter-data-rest</artifactId>
 		</dependency>
 		<dependency>
-			<groupId>org.hsqldb</groupId>
-			<artifactId>hsqldb</artifactId>
+			<groupId>com.h2database</groupId>
+			<artifactId>h2</artifactId>
 			<scope>runtime</scope>
 		</dependency>
 		<dependency>

spring-boot-samples/spring-boot-sample-data-rest/src/test/resources/application-scratch.properties

-spring.datasource.url: jdbc:hsqldb:mem:restdb
+spring.datasource.url: jdbc:h2:mem:restdb