Fix Jacoco generation for Java. (#82)

Author: vanniktech

src/main/groovy/com/vanniktech/android/junit/jacoco/GenerationPlugin.groovy

@@ -63,8 +63,8 @@ class GenerationPlugin implements Plugin<Project> {
             }
 
             classDirectories = subProject.fileTree(
-                    dir: 'build/classes/main/',
-                    excludes: getExcludes(extension)
+                    dir: 'build/classes/', // Starting from Gradle 4.+ each language has it's own classes folder like build/classes/java/main or build/classes/java/test.
+                    excludes: getExcludes(extension) + '**/test/**' // We'll exclude the test directory.
             )
 
             final def coverageSourceDirs = [

src/test/groovy/com/vanniktech/android/junit/jacoco/GenerationTest.groovy

@@ -263,7 +263,7 @@ public class GenerationTest {
               assert sourceDirectories.contains(project.file("src/main/$it"))
             }
 
-            assert classDirectories.dir == project.file('build/classes/main/')
+            assert classDirectories.dir == project.file('build/classes/')
 
             assert reports.xml.enabled
             assert reports.html.enabled