title: shadow plugin in gradle is not working - gradle build does not build a fat jar
body: <p>I have the following <code>build.gradle</code> content:</p>
  
  <pre><code>group 'com.example'
  version '1.0-SNAPSHOT'
  
  buildscript {
      repositories {
          mavenLocal()
          mavenCentral()
          jcenter()
          maven { url "https://plugins.gradle.org/m2/" }
      }
      dependencies {
          classpath "com.github.jengelman.gradle.plugins:shadow:2.0.1"
      }
  }
  
  
  apply plugin: 'scala'   
  apply plugin: "com.github.johnrengelman.shadow"
  sourceCompatibility = java_version
  
  
  repositories {
      mavenCentral()
  }
  
  dependencies {
      // my dependencies:
      compile group: 'org.apache.spark', name: "spark-sql_$scala_major", version: spark_version
      testCompile group: 'junit', name: 'junit', version: '4.12'
  }
  </code></pre>
  
  <p>The problem is that running <code>gradle build</code> isn't creating a fat jar.</p>
  
  <p>Looking at the task dependencies by running <code>gradle tasks --all</code> shows that the <code>shadowJar</code> task exists, but the <code>build</code> task is not dependent on it:</p>
  
  <pre><code>:tasks
  
  ------------------------------------------------------------
  All tasks runnable from root project
  ------------------------------------------------------------
  
  Build tasks
  -----------
  assemble - Assembles the outputs of this project.
  build - Assembles and tests this project.
  buildDependents - Assembles and tests this project and all projects that depend on it.
  buildNeeded - Assembles and tests this project and all projects it depends on.
  classes - Assembles main classes.
  clean - Deletes the build directory.
  jar - Assembles a jar archive containing the main classes.
  testClasses - Assembles test classes.
  
  Build Setup tasks
  -----------------
  init - Initializes a new Gradle build.
  wrapper - Generates Gradle wrapper files.
  
  Documentation tasks
  -------------------
  javadoc - Generates Javadoc API documentation for the main source code.
  scaladoc - Generates Scaladoc for the main source code.
  
  Help tasks
  ----------
  buildEnvironment - Displays all buildscript dependencies declared in root project 'scalaGradleHW'.
  components - Displays the components produced by root project 'scalaGradleHW'. [incubating]
  dependencies - Displays all dependencies declared in root project 'scalaGradleHW'.
  dependencyInsight - Displays the insight into a specific dependency in root project 'scalaGradleHW'.
  dependentComponents - Displays the dependent components of components in root project 'scalaGradleHW'. [incubating]
  help - Displays a help message.
  model - Displays the configuration model of root project 'scalaGradleHW'. [incubating]
  projects - Displays the sub-projects of root project 'scalaGradleHW'.
  properties - Displays the properties of root project 'scalaGradleHW'.
  tasks - Displays the tasks runnable from root project 'scalaGradleHW'.
  
  Shadow tasks
  ------------
  knows - Do you know who knows?
  shadowJar - Create a combined JAR of project and runtime dependencies
  
  Verification tasks
  ------------------
  check - Runs all checks.
  test - Runs the unit tests.
  
  Other tasks
  -----------
  compileJava - Compiles main Java source.
  compileScala - Compiles the main Scala source.
  compileTestJava - Compiles test Java source.
  compileTestScala - Compiles the test Scala source.
  processResources - Processes main resources.
  processTestResources - Processes test resources.
  </code></pre>