Switch Postgres driver dependency from test to optional dependency.

See #1567
Original pull request: #1570

Author: mp911de

spring-data-jdbc/pom.xml

@@ -122,7 +122,7 @@
 			<groupId>org.postgresql</groupId>
 			<artifactId>postgresql</artifactId>
 			<version>${postgresql.version}</version>
-			<scope>test</scope>
+			<optional>true</optional>
 		</dependency>
 
 		<dependency>