Fix Javadoc @Since checkstle violations

Apply consistent `@Since` tags and retrofit the existing code using a
best guess from the git history.

Closes gh-17360

spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootContextLoader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -75,6 +75,7 @@ import org.springframework.web.context.support.GenericWebApplicationContext;
  * @author Phillip Webb
  * @author Andy Wilkinson
  * @author Stephane Nicoll
+ * @since 1.4.0
  * @see SpringBootTest
  */
 public class SpringBootContextLoader extends AbstractContextLoader {