[Java] Upgrade to Checkstyle 8.1.

Author: mjpt777

build.gradle

@@ -118,7 +118,7 @@ subprojects {
     [compileJava, compileTestJava]*.options*.encoding = 'UTF-8'
 
     dependencies {
-        checkstyle 'com.puppycrawl.tools:checkstyle:8.0'
+        checkstyle 'com.puppycrawl.tools:checkstyle:8.1'
         checkstyle 'com.github.sevntu-checkstyle:sevntu-checks:1.24.1'
 
         testCompile 'org.hamcrest:hamcrest-all:1.3'
@@ -129,7 +129,7 @@ subprojects {
 
     checkstyle {
         configFile = new File(rootDir, 'config/checkstyle/checkstyle.xml')
-        toolVersion = '8.0'
+        toolVersion = '8.1'
     }
 
     compileJava {