Greetings.

Unusual request, but I was able to build the Library for Java 1.6 with

java {
 sourceCompatibility = '1.8'
 targetCompatibility = '1.6'
}

Please try the attached JAR file, it should be Java 1.6 compatible -- but I can't try that myself because I was not born yet when 1.6 was available :-D So if it does not work, then you will be on your own.
JSQLParser-4.7-SNAPSHOT-Java6.zip

Since it uses lambdas and streaming the target compatibility is not enough. You are not able to build a version that uses JDK 1.6 API without refactoring. Version 1.2 switched to JDK 1.7. So this should be useable with Jdk 1.6. V3 made the switch to JDK 1.8.