Fix some warns on nightly

[niklasad1]

warning: a method with this name may be added to the standard library in the future
   --> ethcore/src/engines/authority_round/mod.rs:718:56
    |
718 |         let remaining = self.step.inner.duration_remaining().as_millis();
    |                                                              ^^^^^^^^^
    |
    = note: #[warn(unstable_name_collisions)] on by default
    = warning: once this method is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = help: call with fully qualified syntax `engines::authority_round::AsMillis::as_millis(...)` to keep using the current method
    = note: add #![feature(duration_as_u128)] to the crate attributes to enable `std::time::Duration::as_millis`

warning: a method with this name may be added to the standard library in the future
   --> ethcore/src/engines/authority_round/mod.rs:728:47
    |
728 |             while self.step.inner.duration_remaining().as_millis() == 0 {
    |                                                        ^^^^^^^^^
    |
    = warning: once this method is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = help: call with fully qualified syntax `engines::authority_round::AsMillis::as_millis(...)` to keep using the current method
    = note: add #![feature(duration_as_u128)] to the crate attributes to enable `std::time::Duration::as_millis`

warning: a method with this name may be added to the standard library in the future
   --> ethcore/src/engines/authority_round/mod.rs:738:59
    |
738 |             let next_run_at = self.step.inner.duration_remaining().as_millis() >> 2;
    |                                                                    ^^^^^^^^^
    |
    = warning: once this method is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = help: call with fully qualified syntax `engines::authority_round::AsMillis::as_millis(...)` to keep using the current method
    = note: add #![feature(duration_as_u128)] to the crate attributes to enable `std::time::Duration::as_millis`