Raise lower limit of futures version (#89)

* Update the version of futures

The `sync` module of the `futures` crate was added in version `0.1.4` so we need to raise the minimum version.

* Update version of futures

Need the fix for the `Clone` bound on the type parameter on `UnboundedSender`, applied in `futures` version `0.1.6`

client/Cargo.toml

@@ -49,7 +49,7 @@ backtrace = "^0.2.1"
 chrono = "^0.2.21"
 data-encoding = "^1.1.2"
 error-chain = "0.1.12"
-futures = "^0.1"
+futures = "^0.1.6"
 lazy_static = "^0.2.1"
 log = "^0.3.5"
 mio = { version = "^0.5.1", optional = true }