Build broken for default-features=false

[phil-opp]

Hi! First of all, thanks a lot for creating this library. It seems like a great fit for embedded and OS development.

Unfortunately, it no longer compiles with default-features=false, since
ManagedSlice::Owned is only available with use_std or use_collections.

error: no associated item named `Owned` found for type `managed::ManagedSlice<'_, _>` in the current scope
  --> /…/smoltcp-ebf9e93b1271bd34/874d192/src/socket/set.rs:64:13
   |
64 |             ManagedSlice::Owned(ref mut sockets) => {
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

(I think it would be a good idea to add an additional travis build config for default-features=false to prevent similar issues in the future.)

[whitequark]

Fixed

[phil-opp]

Thanks!