[maven-release-plugin] prepare for next development iteration

Author: PhantomThief

pom.xml

@@ -2,7 +2,7 @@
     <modelVersion>4.0.0</modelVersion>
     <groupId>com.github.phantomthief</groupId>
     <artifactId>buffer-trigger</artifactId>
-    <version>0.2.20</version>
+    <version>0.2.21-SNAPSHOT</version>
 
     <prerequisites>
         <maven>3.0.4</maven>