Add explicit dependency for doxia-sink-api 1.2

maven-reporting-impl requires doxia-sink-api 1.2, but it seems that
maven-reporting-api's dependency gives us doxia-sink-api 1.0 on
Maven 3.0.3.

Use an explicit dependency on the newer doxia-sink as a workaround.
Merge pull request #20 from dhurrell/doxia-dependency

Add explicit dependency for doxia-sink-api 1.2