Skip Tattletale's generation of the .png file

As Marek found, Tattletale may spend very long time creating the .PNG graph of the dependencies (using `dot ...`).
The resulting PNG is unusable anyway - too many lines.

Here's the class creating the PNG: https://github.com/jesperpedersen/tattletale/blob/master/src/main/java/org/jboss/tattletale/reporting/GraphvizReport.java

We should figure out how to disable this:

enableDot = Boolean.valueOf(config.getProperty("enableDot", "true"));