Bump jacoco/checkstyle versions

Author: acoburn

build.gradle

@@ -37,9 +37,9 @@ ext {
     /* Testing */
     activationVersion = '1.2.0'
     apiguardianVersion = '1.0.0'
-    checkstyleVersion = '8.12'
+    checkstyleVersion = '8.18'
     jaxbVersion = '2.3.1'
-    jacocoVersion = '0.8.2'
+    jacocoVersion = '0.8.3'
     junitPlatformVersion = '1.4.1'
     junitVersion = '5.4.1'
     logbackVersion = '1.2.3'