Last step to install retrolambda... that github readme didn't tell you

 String getJavaHome(String version)  
 {  
   def stdout = new ByteArrayOutputStream()  
   exec {  
     commandLine "/usr/libexec/java_home", "-v", version  
     standardOutput = stdout;  
   }  
   return stdout.toString().trim()  
 }  
 retrolambda {  
   jdk System.getenv("JAVA8_HOME") ?: getJavaHome("1.8")  
   oldJdk System.getenv("JAVA7_HOME") ?: getJavaHome("1.7")  
   // ...  
 }  

source : https://gist.github.com/Takhion/5c0f6c0c5aba9db5a488