Update POM for 0.4.21 release.

Author: coobird

pom.xml

@@ -2,7 +2,7 @@
   <modelVersion>4.0.0</modelVersion>
   <groupId>net.coobird</groupId>
   <artifactId>thumbnailator</artifactId>
-  <version>0.4.21-SNAPSHOT</version>
+  <version>0.4.21</version>
   <packaging>jar</packaging>
   <name>thumbnailator</name>
   <description>Thumbnailator - a thumbnail generation library for Java</description>