Revert "Upgrade to Groovy 3.0.3"

This reverts commit ff65ccba3e.

Compiling against Groovy 3.0.3 currently results in the following two
warnings that cause the build to fail.

warning: Cannot find annotation method 'value()' in type 'SuppressFBWarnings': class file for edu.umd.cs.findbugs.annotations.SuppressFBWarnings not found

warning: Cannot find annotation method 'justification()' in type 'SuppressFBWarnings'

build.gradle

@@ -50,7 +50,7 @@ configure(allprojects) { project ->
 				entry 'aspectjtools'
 				entry 'aspectjweaver'
 			}
-			dependencySet(group: 'org.codehaus.groovy', version: '3.0.3') {
+			dependencySet(group: 'org.codehaus.groovy', version: '2.5.11') {
 				entry 'groovy'
 				entry 'groovy-jsr223'
 				entry 'groovy-templates'