Fix outdated riemann_client dependency

[bpetit]

Bug description

Riemann exporter now relies on riemann_client 0.9, which is flagged as outdated and should be replaced.

The riemann_client project seems inactive, we should consider

To Reproduce

Compile scaphandre with cargo build, see the error :

warning: the following packages contain code that will be rejected by a future version of Rust: riemann_client v0.9.0
note: to see what the problems were, use the option --future-incompat-report, or run cargo report future-incompatibilities --id 1

Expected behavior

No warning, use an up to date dependency.

Additional context

Pinging @uggla if available as he knows the exporter very well.