Fixed an issue where the sourceCompatibility and targetCompatibility options were not being applied properly

[aeharvlee]

Proposed changes

This PR fixed an issue where the sourceCompatibility and targetCompatibility options were not being applied properly.
To make sourceCompatibility, targetCompatibility works, we need to define it below apply plugin: 'java'.

Here is the guide "How to check whether caver-java library is built with JDK 1.8 (= Java 8)"

1. Get into circle ci machine using ssh
2. After build step is finished, move to /home/circleci/circleci-caver-java-android/core/build/libs
3. Extract class files from jar using jar xf core-1.6.1-android.jar
4. Check which version of JVM is used to compile class files using javap -verbose /home/circleci/circleci-caver-java-android/core/build/libs/com/klaytn/caver/Caver.class | grep "major" (You can check another lib like codegen or console like this)

If the result is major version: 52, everything is OK.
Below is a version info about Java.

• Java 8 uses major version 52
• Java 9 uses major version 53
• Java 10 uses major version 54
• Java 11 uses major version 55

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