Set version to 1.21-SNAPSHOT

Change-Id: I8a630a8a5287c6af964ef6d5bed3017a8009b3f3

pom.xml

diff --git a/pom.xml b/pom.xml
index 16d65cb..de72eea 100644
--- a/pom.xml
+++ b/pom.xml
@@ -21,7 +21,7 @@
   <groupId>com.google.gerrit</groupId>
   <artifactId>gwtorm</artifactId>
   <packaging>jar</packaging>
-  <version>1.20</version>
+  <version>1.21-SNAPSHOT</version>
   <name>gwtorm</name>
   <description>Tiny ORM</description>
   <url>https://gerrit.googlesource.com/gwtorm</url>