ingore jar files

JRuby's DLEXT is "jar", so compilation of the C++ library w/ JRuby
creates the so file as dep_gecode.jar (it's not actually a jar). This
doesn't cause any runtime issues (though it's weird).
Lock ffi to ~> 1.9

This is based on 1.9.3 being the version I tested with. If this causes
conflicts with other projects, we can change this as long as we make
sure it all works.