Update to Spring Security 4.2.15

Closes gh-556

Author: rwinch

gradle/java.gradle

@@ -23,7 +23,7 @@ ext.powerMockVersion = '1.6.5'
 ext.commonsPool2Version = '2.4.2'
 ext.assertjVersion= '2.3.0'
 ext.hibernateVersion = '5.2.5.Final'
-ext.springSecurityVersion='4.2.0.RELEASE'
+ext.springSecurityVersion='4.2.15.RELEASE'
 ext.unboundidVersion = '3.1.1'
 
 ext.powerMockDependencies = [