Bump tokio from 0.2.11 to 0.2.12

Bumps [tokio](https://github.com/tokio-rs/tokio) from 0.2.11 to 0.2.12.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](https://github.com/tokio-rs/tokio/compare/tokio-0.2.11...tokio-timer-0.2.12)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Cargo.lock

@@ -1645,9 +1645,9 @@ dependencies = [
 
 [[package]]
 name = "tokio"
-version = "0.2.11"
+version = "0.2.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8fdd17989496f49cdc57978c96f0c9fe5e4a58a8bddc6813c449a4624f6a030b"
+checksum = "b34bee1facdc352fba10c9c58b654e6ecb6a2250167772bf86071f7c5f2f5061"
 dependencies = [
  "bytes",
  "fnv",