Parameterize spring_version in the README for javadoc links

Author: gregturn

README.adoc

@@ -1,8 +1,9 @@
+:spring_version: 4.0.0.RC1
 :spring_boot_version: 0.5.0.M6
 :SpringSocial: http://projects.spring.io/spring-social/
-:Component: http://docs.spring.io/spring/docs/4.0.0.RC1/javadoc-api/org/springframework/stereotype/Component.html
+:Component: http://docs.spring.io/spring/docs/{spring_version}/javadoc-api/org/springframework/stereotype/Component.html
 :EnableAutoConfiguration: http://docs.spring.io/spring-boot/docs/{spring_boot_version}/api/org/springframework/boot/autoconfigure/EnableAutoConfiguration.html
-:DispatcherServlet: http://docs.spring.io/spring/docs/4.0.0.RC1/javadoc-api/org/springframework/web/servlet/DispatcherServlet.html
+:DispatcherServlet: http://docs.spring.io/spring/docs/{spring_version}/javadoc-api/org/springframework/web/servlet/DispatcherServlet.html
 :SpringApplication: http://docs.spring.io/spring-boot/docs/{spring_boot_version}/api/org/springframework/boot/SpringApplication.html
 :gs-register-twitter-app: link:/guides/gs/register-twitter-app
 :toc: