There are a couple of legacy dependencies that Activiti current depends on:
1. commons-lang 2.4 (could be upgraded to commons-lang3 3.1)
2. jackson 1.99 (could be upgraded to 2.2.2)
These legacy dependencies lead to duplicate JAR files in projects embedding Activiti, like as Apache Syncope.
In fact, both updated versions changed their base package (org.apache.commons.lang -> org.apache.commons.lang3 and org.codehaus.jackson -> com.fasterxml.jackson) so some changes in the code are needed as well.
I have already provided a pull request  for commons-lang3 but lately I have also found another pull request  for the same topic.
If this is of interest, I can volunteer for providing a pull request for jackson as well.