Re-enable publication of coverage reports

Prior to this commit, jacoco coverage report XML files were not actually
being written to disk, meaning that the Gradle coveralls plugin had
nothing to publish to https://coveralls.io/r/ethereum/ethereumj. HTML
reports are now being written as well, for convenient local browsing.

.travis.yml

@@ -7,6 +7,7 @@ env:
 script:
   - ./gradlew build publish
 
+# publish coverage report to https://coveralls.io/r/ethereum/ethereumj
 after_success:
   - ./gradlew jacocoTestReport coveralls
 

ethereumj-core/build.gradle

@@ -98,6 +98,13 @@ artifacts {
     archives javadocJar
 }
 
+jacocoTestReport {
+    reports {
+        xml.enabled = true
+        html.enabled = true
+    }
+}
+
 publishing {
     publications {
         mavenJava(MavenPublication) {