Replace read_string_length with read_string with length argument for JRuby.
Replace read_string_length with read_string with length argument for JRuby.

JRuby doesn't have a separate read_string_length, but read_string with
a length argument works for all supported Rubies.

mvz/gir_ffi,jcupitt/gir_ffi,jcupitt/gir_ffi,mvz/gir_ffi,mvz/gir_ffi