chore(build.gradle): update java version to 8

Author: kevinkowa

build.gradle

@@ -63,8 +63,8 @@ allprojects {
   apply plugin: 'jacoco'
   apply plugin: 'com.github.johnrengelman.shadow'
 
-  sourceCompatibility = JavaVersion.VERSION_1_7
-  targetCompatibility = JavaVersion.VERSION_1_7
+  sourceCompatibility = JavaVersion.VERSION_1_8
+  targetCompatibility = JavaVersion.VERSION_1_8
 
   repositories {
     maven { url "https://repo.maven.apache.org/maven2" }