gpu-model compile failure

[YuqiHUO]

when I complie from the source, I use the latest master branch. When make, it shows the error
[ 2%] Building CXX object CMakeFiles/decord.dir/src/video/video_reader.cc.o
/.../decord/src/video/video_reader.cc: In member function ‘virtual void decord::VideoReader::SetVideoStream(int)’:
/.../decord/src/video/video_reader.cc:101:62: error: invalid new-expression of abstract class type ‘decord::cuda::CUThreadedDecoder’
ctx_.device_id, codecpar.get(), fmt_ctx_->iformat));

may it be the code's problem?

[zhreshold]

Should be fixed by #58