Update gradle to version 6.1.1

Change-Id: I09f57b0e6cca43563ec098ece4089704f14940e0
diff --git a/build.gradle b/build.gradle
index 84bb03c..fe341fc 100644
--- a/build.gradle
+++ b/build.gradle
@@ -11,8 +11,8 @@
   id 'maven'
   id 'signing'
   id 'checkstyle'
-  id 'org.sonarqube' version '2.7.1'
-  id 'net.saliman.cobertura' version '2.6.1'
+  id 'org.sonarqube' version '2.8'
+  id 'net.saliman.cobertura' version '3.0.0'
 }
 
 group = 'net.named-data.jndn-extra'