Use correct maven dependency coordinates for caja

Shindig has a maven dependency to google's caja defined in java/gadgets/pom.xml (and in <dependencyManagement of the main pom.xml):

<dependency>
<groupId>caja</groupId>
<artifactId>caja</artifactId>
<version>r5054</version>
</dependency>

According to caja's POM the groupId should be com.google.caja instead of caja. But as this caja artifact currently is not available from a valid repository, caja's issue #1938 (http://code.google.com/p/google-caja/issues/detail?id=1938) must be fixed earlier.

The whole problem was discussed in this thread on the dev-list:
http://mail-archives.apache.org/mod_mbox/incubator-shindig-dev/201409.mbox/browser