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-samples/spring-boot-sample-tomcat/src/test/java/sample/tomcat/NonAutoConfigurationSampleTomcatApplicationTests.java

@@ -50,7 +50,7 @@ import static org.junit.Assert.assertEquals;
  * @author Dave Syer
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = NonAutoConfigurationSampleTomcatApplication.class)
+@SpringApplicationConfiguration(NonAutoConfigurationSampleTomcatApplication.class)
 @WebAppConfiguration
 @IntegrationTest("server.port=0")
 @DirtiesContext

spring-boot-samples/spring-boot-sample-tomcat/src/test/java/sample/tomcat/SampleTomcatApplicationTests.java

@@ -45,7 +45,7 @@ import static org.junit.Assert.assertEquals;
  * @author Andy Wilkinson
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleTomcatApplication.class)
+@SpringApplicationConfiguration(SampleTomcatApplication.class)
 @WebIntegrationTest(randomPort = true)
 @DirtiesContext
 public class SampleTomcatApplicationTests {