backward compatibility for ≤ R_3.4.4

Reading from connections in text mode is buffered for >= R_3.5.0.
No use buff in the new version (>=3.5.0) of R_ext/Connections.h:

struct Rconn {
    ...
    unsigned char *buff;
    size_t buff_len, buff_stored_len, buff_pos;
};

Install:

library(devtools)
install_github("zhengxwen/SeqArray", ref="1d5ab05fa8ae8b754feab62f41ab00a182d54793")