Upgrade to Freemarker 2.3.21

Closes gh-1880

Author: wilkinsona

spring-boot-dependencies/pom.xml

@@ -57,7 +57,7 @@
 		<commons-pool2.version>2.2</commons-pool2.version>
 		<crashub.version>1.3.0</crashub.version>
 		<flyway.version>3.0</flyway.version>
-		<freemarker.version>2.3.20</freemarker.version>
+		<freemarker.version>2.3.21</freemarker.version>
 		<gemfire.version>7.0.2</gemfire.version>
 		<gradle.version>1.6</gradle.version>
 		<groovy.version>2.3.7</groovy.version>