Fix feature name tokio->use_tokio in travis config

[pheki]

After finding the regression I fixed in #61 I was very surprised that travis managed to compile #60 just fine on macOS (stable) and tried building it on my machine (which failed).

Then I realised that the config file was using --features tokio instead of use_tokio.

[rust-highfive]

r? @posborne

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[nastevens]

I also built #60 on a Linux box but forgot the use_tokio feature. Should we maybe change this to --all-features in CI to avoid this problem in the future? On second thought, it looks like CI is specifically using the features to make sure that a normal build (without tokio) will still build. So I think what you have here is good 👍