Upgrade guide to indicate requirement for Java 7

README.adoc

@@ -18,6 +18,7 @@ You'll build a simple web application with Spring Boot and add some useful servi
 
 == What you'll need
 
+:java_version: 1.7
 include::https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/prereq_editor_jdk_buildtools.adoc[]
 
 include::https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/how_to_complete_this_guide.adoc[]