Add raptor_world arg to FFI definition for func raptor_free_world (instead of no args)

[emgullufsen-lii]

The underlying c library seems to accept a pointer to a raptor_world object, whereas the FFI defines no args to this function.

Here is the library definition:
https://github.com/dajobe/raptor/blob/e4285aefef442da7e4e847dfd77097a40506773f/src/raptor2.h.in#L1051

void raptor_free_world(raptor_world* world);

But here we have no args defined:

rdf-raptor/lib/rdf/raptor/ffi/v2.rb

Line 33 in 76411d5

attach_function :raptor_free_world, [], :void

Please see this issue for more detailed explanation:
#27

Fixes #27.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

coverage: 79.315%. remained the same
when pulling fef3343 on emgullufsen-lii:develop
into 76411d5 on ruby-rdf:develop.

[gkellogg]

Thanks!

[emgullufsen-lii]

Thanks for the gem!