The GoJara plugin uses a library named json.jar. This library was compiled in a fashion incompatible with Java 5. This causes compilation to fail (Openfire requires Java 5, not 6):
[javac] /opt/j2ee/domains/igniterealtime.org/bamboo/webapps/atlassian-bamboo/data/atlassian-bamboo-4.2.0/xml-data/build-dir/OPENFIRE-NIGHTLYPLUGINS-JOB1/src/plugins/gojara/src/java/org/jivesoftware/openfire/plugin/gojara/servlets/StatisticsServlet.java:13: cannot access org.json.JSONArray
[javac] bad class file: /opt/j2ee/domains/igniterealtime.org/bamboo/webapps/atlassian-bamboo/data/atlassian-bamboo-4.2.0/xml-data/build-dir/OPENFIRE-NIGHTLYPLUGINS-JOB1/src/plugins/gojara/lib/json.jar(org/json/JSONArray.class)
[javac] class file has wrong version 50.0, should be 49.0
[javac] Please remove or make sure it appears in the correct subdirectory of the classpath.
[javac] import org.json.JSONArray;
[javac] 1 error
[trycatch] Caught exception: Compile failed; see the compiler error output for details.
The offending jar should be replaced with a Java 5 compatible equivalent (with a bit of luck, it needs recompilation only).