[Java] Upgrade to Checkstyle 6.19.

Author: mjpt777

build.gradle

@@ -115,7 +115,7 @@ subprojects {
     [compileJava, compileTestJava]*.options*.encoding = 'UTF-8'
 
     dependencies {
-        checkstyle "com.github.sevntu.checkstyle:sevntu-checks:1.20.0"
+        checkstyle "com.github.sevntu.checkstyle:sevntu-checks:1.21.0"
 
         testCompile 'org.hamcrest:hamcrest-all:1.3'
         testCompile 'junit:junit:4.12'
@@ -125,7 +125,7 @@ subprojects {
 
     checkstyle {
         configFile = new File(rootDir, 'config/checkstyle.xml')
-        toolVersion = "6.18"
+        toolVersion = "6.19"
     }
 
     compileJava {