Update public version

web interface still shows version 3.0.0, even though 3.1.0 is installed.

Author: Ryan Moore

src/main/java/org/graylog2/plugins/slack/SlackPluginMetadata.java

@@ -32,7 +32,7 @@ public URI getURL() {
 
     @Override
     public Version getVersion() {
-        return new Version(3, 0, 0);
+        return new Version(3, 1, 0);
     }
 
     @Override