Read request-bodies from /resources/request-bodies

I added the ```-bf``` option so that Gatling can read request bodies from within the project.


Does this give gradle warnings?  I think referencing an undeclared variable is taking advantage of a deprecated feature in gradle (can't remember details though..).  Either way, declaring this `final` would be in keeping with the surrounding code.  (I think there's no need for `def` if you have `final`.)