Bump to 0.8.13

This release incorporates significant improvements to cernan and is
a recommented upgrade. In particular, cernan now uses hopper 0.4.
This update of hopper is _significantly_ more efficient, being only
about 3x slower than built-in MPSC while maintaining resource
quotas. These quotas are now exposed as configuration to cernan. An
experimental kafka sink is also added but care should be taken with
its use. Mike Lang has been landing improvements to the programmable
filters and it is hoped they are now more useful.

Cernan's baseline acceptance test is now 272k statsd PPS, held in
10Mb. This is up from 45k statsd PPS at 150Mb.

  * dcbeb7d :: Reduce high PPS memory consumption
  * 4d2d118 :: Update hopper to 0.4
  * d10d195 :: Added json_encode filter
  * ecc2fdf :: Kafka sink introduced

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

Cargo.toml

@@ -8,7 +8,7 @@ license = "MIT"
 name = "cernan"
 readme = "README.md"
 repository = "https://github.com/postmates/cernan"
-version = "0.8.13-pre"
+version = "0.8.13"
 
 [[bin]]
 name = "cernan"