Fix build error on rust 1.27

[yihuang]

rust-lang/rust#49502

[NicolasDP]

thanks for looking into it. Can you tell us more why you need this specific version of the rust compiler ?

My concern is that SystemTime::UNIX_EPOCH is a recent change and that it might deprecates the former way to get this value.

Also, there is a serious security concern regarding all compilers prior 1.29.1 and I can only recommend you update the compiler version you depend on.

[mzabaluev]

From discussion on the library change, the issue was more about discoverability of the constant, and the old name is OK otherwise. In any case, it has to stay there for Rust 1.0 timeframe.

That said, we may prefer to only target new compiler versions in the code, and switch to the 2018 edition as the baseline when it gets released.

[yihuang]

It doesn't have the problem with the recent version of rust, my version was too old.

[NicolasDP]

Thanks @yihuang , I think we can close this PR then if it is just easy to fix with an update of the compiler.