Update Gradle build to use the Kotlin DSL

Fix gh-33

build.gradle.kts

@@ -0,0 +1,55 @@
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
+plugins {
+	kotlin("plugin.jpa") version "1.2.71"
+	id("org.springframework.boot") version "2.1.5.RELEASE"
+	id("io.spring.dependency-management") version "1.0.7.RELEASE"
+	kotlin("jvm") version "1.2.71"
+	kotlin("plugin.spring") version "1.2.71"
+	kotlin("plugin.allopen") version "1.2.71"
+	kotlin("kapt") version "1.2.71"
+}
+
+group = "com.example"
+version = "0.0.1-SNAPSHOT"
+java.sourceCompatibility = JavaVersion.VERSION_1_8
+
+repositories {
+	mavenCentral()
+}
+
+dependencies {
+	implementation("org.springframework.boot:spring-boot-starter-data-jpa")
+	implementation("org.springframework.boot:spring-boot-starter-mustache")
+	implementation("org.springframework.boot:spring-boot-starter-web")
+	implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
+	implementation("org.jetbrains.kotlin:kotlin-reflect")
+	implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
+	runtimeOnly("com.h2database:h2:1.4.197") // Fixed version as a workaround for https://github.com/h2database/h2database/issues/1841
+	runtimeOnly("org.springframework.boot:spring-boot-devtools")
+	kapt("org.springframework.boot:spring-boot-configuration-processor")
+	testImplementation("org.springframework.boot:spring-boot-starter-test") {
+		exclude(module = "junit")
+		exclude(module = "mockito-core")
+	}
+	testImplementation("org.junit.jupiter:junit-jupiter-api")
+	testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
+	testImplementation("com.ninja-squad:springmockk:1.1.2")
+}
+
+tasks.withType<KotlinCompile> {
+	kotlinOptions {
+		freeCompilerArgs = listOf("-Xjsr305=strict")
+		jvmTarget = "1.8"
+	}
+}
+
+allOpen {
+	annotation("javax.persistence.Entity")
+	annotation("javax.persistence.Embeddable")
+	annotation("javax.persistence.MappedSuperclass")
+}
+
+tasks.withType<Test> {
+	useJUnitPlatform()
+}