Do not produce output if the PhpSpec output format is junit or html

I created this as we use the junit output PhpSpec provides, however the output this extension produces makes the generated xml invalid.

Looks like PhpSpec will provide an `--output` parameter for writing their test results to a file directly somewhere in the future. However for now this seems like a simple fix, as I can't think of a reason for writing that line when PhpSpec is generating xml/html.
That's also an acceptable solution for me, I'll change it :)