appveyor: use LuaCov-coveralls with dkjson
appveyor: use LuaCov-coveralls with dkjson

lua-cjson fails to compile in AppVeyor (see build [1]).

[1] https://ci.appveyor.com/project/starius/lua-npge/build/0.0.1.29-test/job/41oqc88o1c2x6x7n#L698

starius/lua-npge,npge/lua-npge,npge/lua-npge,starius/lua-npge,npge/lua-npge,starius/lua-npge