Oracle hasn't made any public announcement about new Java certification exam on Java version 8. Because you are planning to write the exam this June/ July, I would recommend that you go for it, rather than waiting for the new version of this exam.
Even if Java 8 is released, I highly doubt that the exam for Java 8 will be released any time 2013. Even 2014 would be a stretch because they just released the Java 7 exam. But with Oracle, you never know