Task dependency order & JDK version used for compilation

These commits are related to two found bugs.

* task build order was incorrect, so Test classes weren't converted to old **class** file format
* both **compilation** and execution of **retrolambda** was performed with JDK used to launch Gradle (the one from **PATH** or **JAVA_HOME**) but not with specified in **project.retrolambda.jdk**


**TODO:** launching tests using old JDK