Upgrade tokio runtime to 0.2

[alun]

It would be nice to upgrade used tokio runtime to 0.2. Currently it is impossible to seamlessly run async code with e.g. latest versions of:

reqwest = "0.10.6"
redis = "0.16.0"
etcd-client = "0.5.2"

They all depend on tokio 0.2 and panic when called from within the bot's async stream which is running on 0.1.

It is possible to workaround by creating 0.2 runtime and use .block_on for futures requiring it but this is obviously far from being perfect.