Add value alias for SpringApplicationConfiguration

Given that Spring Boot uses java config accross the board, a new `value`
attribute is now aliased to the existing `classes` attribute such that
one could write the following:

@SpringApplicationConfiguration(MyConfig.class)
public class MyTest {}

Closes gh-3635

spring-boot-cli/test-samples/integration.groovy

@@ -1,4 +1,4 @@
-@SpringApplicationConfiguration(classes=Application)
+@SpringApplicationConfiguration(Application)
 @IntegrationTest
 class BookTests {
 	@Autowired

spring-boot-cli/test-samples/integration_auto.groovy

@@ -1,4 +1,4 @@
-@SpringApplicationConfiguration(classes=Application)
+@SpringApplicationConfiguration(Application)
 @IntegrationTest('server.port:0')
 @WebAppConfiguration
 @DirtiesContext

spring-boot-cli/test-samples/integration_auto_test.groovy

@@ -1,4 +1,4 @@
-@SpringApplicationConfiguration(classes=ReactorApplication)
+@SpringApplicationConfiguration(ReactorApplication)
 @IntegrationTest('server.port:0')
 class RestTests {