Update README.md

Closes #579

Author: jschueller

README.md

@@ -14,5 +14,5 @@ To build it refer to the [installation instructions](https://nlopt.readthedocs.i
 To use in your C/C++ programs see the [C reference manual](https://nlopt.readthedocs.io/en/latest/NLopt_Reference/).
 
 There are also interfaces for [C++](https://nlopt.readthedocs.io/en/latest/NLopt_C-plus-plus_Reference/), [Fortran](https://nlopt.readthedocs.io/en/latest/NLopt_Fortran_Reference/), [Python](https://nlopt.readthedocs.io/en/latest/NLopt_Python_Reference/), [Matlab or GNU Octave](https://nlopt.readthedocs.io/en/latest/NLopt_Matlab_Reference/), [OCaml](https://bitbucket.org/mkur/nlopt-ocaml),
-[GNU Guile](https://nlopt.readthedocs.io/en/latest/NLopt_Guile_Reference/), [GNU R](https://www.ucl.ac.uk/~uctpjyy/nloptr.html), [Lua](https://github.com/rochus-keller/LuaNLopt), [Rust](https://github.com/jesskfullwood/rust-nlopt), and [Julia](https://github.com/JuliaOpt/NLopt.jl).  Interfaces for other languages may
+[GNU Guile](https://nlopt.readthedocs.io/en/latest/NLopt_Guile_Reference/), [GNU R](https://www.ucl.ac.uk/~uctpjyy/nloptr.html), [Lua](https://github.com/rochus-keller/LuaNLopt), [Rust](https://github.com/jesskfullwood/rust-nlopt), [Ruby](https://github.com/ankane/nlopt-ruby), and [Julia](https://github.com/JuliaOpt/NLopt.jl).  Interfaces for other languages may
 be added in the future.