Change groupId to com.threerings.getdown.

I don't want to pollute the com.threerings top-level namespace with a bunch of
getdown-foo artifacts, and grouping everything under the top-level project
name, even when that name is repeated for all the artifacts is fairly standard
practice.

core/pom.xml

@@ -2,7 +2,7 @@
 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <parent>
-    <groupId>com.threerings</groupId>
+    <groupId>com.threerings.getdown</groupId>
     <artifactId>getdown</artifactId>
     <version>1.8-SNAPSHOT</version>
   </parent>