How to solve this project run with the error below?

[HUAIHU234]

Error:Error converting bytecode to dex:
Cause: Dex cannot parse version 52 byte code.
This is caused by library dependencies that have been compiled using Java 8 or above.
If you are using the 'java' gradle plugin in a library submodule add
targetCompatibility = '1.7'
sourceCompatibility = '1.7'
to that submodule's build.gradle file.
...while parsing com/facebook/rebound/BaseSpringSystem.class
Error:1 error; aborting
Error:Execution failed for task ':rebound-android-playground:transformClassesWithDexForDebug'.

com.android.build.api.transform.TransformException: com.android.ide.common.process.ProcessException: java.util.concurrent.ExecutionException: com.android.ide.common.process.ProcessException: Error while executing java process with main class com.android.dx.command.Main with arguments {--dex --num-threads=4 --output /Users/huaihuhu/Desktop/rebound/rebound-0.3.8/rebound-android-playground/build/intermediates/transforms/dex/debug/folders/1000/1f/main /Users/huaihuhu/Desktop/rebound/rebound-0.3.8/rebound-core/build/libs/rebound-core.jar /Users/huaihuhu/Desktop/rebound/rebound-0.3.8/rebound-android/build/intermediates/bundles/default/classes.jar /Users/huaihuhu/Desktop/rebound/rebound-0.3.8/rebound-android-playground/build/intermediates/classes/debug}

[colorhaake]

try this PR:
#77
All error would be gone.