NLOPT library

Hello, Tomas.

In future versions of Rtools44, for the time being, please do not
update the NLOPT libraries beyond 2.8.0. There are breaking changes in
2.9.0 and possibly 2.9.1 which cause nloptr to fail. Please see
https://github.com/astamm/nloptr/issues/173 for more details.

Thank you very much,

Avi