Update bytes(v0.4.12), parking_lot_core(0.6.2), toml(0.5.3), tokio(0.1.17)

Update following packages:
bytes v0.4.7 -> v0.4.12
parking_lot_core v0.6.1 -> v0.6.2
toml v0.5.1 -> v0.5.3
tokio v0.1.7 -> v0.1.17

Commands were:
```
cargo update --package bytes
cargo update --package parking_lot_core:0.6.1
cargo update --package toml:0.5.1
cargo update --package tokio:0.1.7 --precise 0.1.17
```

Errors were:
1. bytes: jsonrpc requires ^0.4.8, and bytes were pinned to v0.4.7 for
sendgrid v0.8.1.
2. parking_lot_core: Amanieu/parking_lot#181
3. toml: `forward_to_deserialize_any_helper` was replaced by forward_to_deserialize_any
4. tokio: `Builder::core_threads` was introduced in 0.1.10

Author: foriequal0