Bump version to 3.21

Author: moxie-signal

pom.xml

@@ -21,7 +21,7 @@
         <mockito.version>2.25.1</mockito.version>
 
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <TextSecureServer.version>3.20</TextSecureServer.version>
+        <TextSecureServer.version>3.21</TextSecureServer.version>
     </properties>
 
     <parent>