build.gradle: upgrade shadow plugin to 4.0.4

The current shadow plugin we are using is 2.0.3, which is incompatible
with Gradle 5.2.1 and gives the error below when running shadowJar
task:

    Method com/github/jengelman/gradle/plugins/shadow/internal/DependencyFileCollection.getBuildDependencies()Lorg/gradle/api/tasks/TaskDependency; is abstract

As we are planning to upgrade to Gradle 5.2.1, let's upgrade the shadow
plugin to its latest version that still supports Gradle 4.x, version 4.0.4, to
solve the problem[1].

[1] GradleUp/shadow#447

build.gradle

@@ -9,7 +9,7 @@ plugins {
     id 'jacoco'
     id 'checkstyle'
     id 'com.github.kt3k.coveralls' version '2.4.0'
-    id 'com.github.johnrengelman.shadow' version '2.0.3'
+    id 'com.github.johnrengelman.shadow' version '4.0.4'
     id 'org.asciidoctor.convert' version '1.5.6'
     id 'application'
 }