Reqwest: Update tokio dependency to 0.2

[kitfre]

With the release of tokio 0.2, it seems like a good time to upgrade reqwest to take advantage of the new tokio scheduler.

This might also need downstream updates as well, particularly in hyper and hyper-tls.

A quick and dirty update of the Cargo.toml with tokio: "=0.2" results in :

failed to select a version for `tokio`.
    ... required by package `hyper v0.13.0-alpha.4`
    ... which is depended on by `hyper-tls v0.4.0-alpha.4`
    ... which is depended on by `reqwest v0.10.0-alpha.2`
versions that meet the requirements `= 0.2.0-alpha.6` are: 0.2.0-alpha.6

all possible versions conflict with previously selected packages.

  previously selected package `tokio v0.2.0`
    ... which is depended on by `reqwest v0.10.0-alpha.2`

failed to select a version for `tokio` which could resolve this conflict

[anton-dutov]

Blocked by hyperium/hyper#2025

[asm89]

The hyper issue is resolved, but I believe this still depends on the hyper-tls library getting upgraded as well: https://github.com/hyperium/hyper-tls/pull/51.

[fenhl]

That blocker seems to have been merged.

[seanmonstar]

Merged in #731.