Update jacoco plugin version to 0.8.4

In particular, starting from 0.8.0 private empty constructors are
excluded from the report (see jacoco/jacoco#529)

DEVSIX-3145

Author: Snipx

pom.xml

@@ -78,7 +78,7 @@
     <hsqldb.version>2.3.3</hsqldb.version>
     <integrationtests>com.itextpdf.test.annotations.type.IntegrationTest</integrationtests>
     <itext.legacy.version>5.5.8</itext.legacy.version>
-    <jacoco.version>0.7.9</jacoco.version>
+    <jacoco.version>0.8.4</jacoco.version>
     <java.version>1.7</java.version>
     <javadoc-additionalOptions/>
     <javadoc-link>https://docs.oracle.com/javase/8/docs/api/</javadoc-link>