Upgrade to Groovy 2.4.21

Closes gh-256

Author: snicoll

pom.xml

@@ -40,7 +40,7 @@
 		<assertj.version>3.8.0</assertj.version>
 		<checkstyle.version>8.32</checkstyle.version>
 		<gradle.version>3.4</gradle.version>
-		<groovy.version>2.4.7</groovy.version>
+		<groovy.version>2.4.21</groovy.version>
 		<maven-core.version>3.5.0</maven-core.version>
 		<maven-plugin-api.version>${maven-core.version}</maven-plugin-api.version>
 		<maven-plugin-annotations.version>3.5</maven-plugin-annotations.version>