Make SpringApplicationConfiguration use SpringApplicationContextLoader

The intention in 1.4 is for the deprecated testing functionality to
behave exactly as it did in 1.3. To help with this, this commit
updates SpringApplicationConfiguration to use
SpringApplicationContextLoader as its loader, just as it did in 1.3.

Closes gh-5882

spring-boot-test/src/main/java/org/springframework/boot/test/SpringApplicationConfiguration.java

@@ -35,7 +35,7 @@ import org.springframework.test.context.ContextConfiguration;
  * {@code ApplicationContext} for integration tests.
  * <p>
  * Similar to the standard {@link ContextConfiguration @ContextConfiguration} but uses
- * Spring Boot's {@link SpringBootContextLoader}.
+ * Spring Boot's {@link SpringApplicationContextLoader}.
  *
  * @author Dave Syer
  * @author Sam Brannen
@@ -44,7 +44,7 @@ import org.springframework.test.context.ContextConfiguration;
  * @deprecated as of 1.4 in favor of {@link SpringBootTest} or direct use of
  * {@link SpringBootContextLoader}.
  */
-@ContextConfiguration(loader = SpringBootContextLoader.class)
+@ContextConfiguration(loader = SpringApplicationContextLoader.class)
 @Documented
 @Inherited
 @Retention(RetentionPolicy.RUNTIME)