Update required JDK version to JDK8

Fixes gh-4154.

README.adoc

@@ -29,9 +29,9 @@ In the instructions below, http://vimeo.com/34436402[`./gradlew`] is invoked fro
 a cross-platform, self-contained bootstrap mechanism for the build.
 
 === Prerequisites
-http://help.github.com/set-up-git-redirect[Git] and the http://www.oracle.com/technetwork/java/javase/downloads[JDK7 build].
+http://help.github.com/set-up-git-redirect[Git] and the http://www.oracle.com/technetwork/java/javase/downloads[JDK8 build].
 
-Be sure that your `JAVA_HOME` environment variable points to the `jdk1.7.0` folder extracted from the JDK download.
+Be sure that your `JAVA_HOME` environment variable points to the `jdk1.8.0` folder extracted from the JDK download.
 
 === Check out sources
 [indent=0]