Using the passed jUnit config if set

Just having a look at this to compare the XML JUnit output to another
JUnit/ PhantomJS task I'm using and noticed that as written it seems to
ignore the passed in JUnit configuration and it's impossible to turn
the JUnit reporting on... 

I think this change makes it behave as expected?
Merge pull request #8 from vitch/patch-1

Using the passed jUnit config if set