Fix Itertools::flatten regression soon in nightly.

[Centril]

This fixes a regression (soon in nightly) due to the pending stabilization of Iterator::flatten.
See rust-lang/rust#51511 for details.

[leoyvens]

Hope @ubnt-intrepid still maintains this, I use this lib.

[ubnt-intrepid]

Thank you for reporting the regression and PR.

Currently, I don't use Gurobi optimizer and hence cannot test this patch in my environment. I'll apply this PR soon and update the minor release, but other problems may be still remain.

@leodasvacas Probably I'll not continue to develop this library in the future. Could you fork this project and get the development of the transition to take over? (If necessary, I'll also consider transfering the registered namespace gurobi in crates.io to the succeed developer.)