Improve Gradle build scripts

app/build.gradle

@@ -1,17 +1,3 @@
-plugins {
-    id 'java'
-    id 'org.jetbrains.kotlin.jvm'
-}
-
-group 'io.bartek'
-version '1.0-SNAPSHOT'
-
-sourceCompatibility = 1.8
-
-repositories {
-    mavenCentral()
-}
-
 dependencies {
     compile project(':api')
     compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
@@ -20,13 +6,6 @@ dependencies {
     compile group: 'org.pf4j', name: 'pf4j', version: '3.2.0'
 }
 
-compileKotlin {
-    kotlinOptions.jvmTarget = "1.8"
-}
-compileTestKotlin {
-    kotlinOptions.jvmTarget = "1.8"
-}
-
 jar {
     zip64 true
     manifest {