[SPARK-9094] [PARENT] Increased io.dropwizard.metrics from 3.1.0 to 3.1.2

We are running Spark 1.4.0 in production and ran into problems because after a network hiccup (which happens often in our current environment) no more metrics were reported to graphite leaving us blindfolded about the current state of our spark applications. [This problem](https://github.com/dropwizard/metrics/commit/70559816f1fc3a0a0122b5263d5478ff07396991) was fixed in the current version of the metrics library. We run spark with this change  in production now and have seen no problems. We also had a look at the commit history since 3.1.0 and did not detect any potentially  incompatible changes but many fixes which could potentially help other users as well.

Author: Carl Anders Düvel <c.a.duevel@gmail.com>

Closes #7493 from hackbert/bump-metrics-lib-version and squashes the following commits:

6677565 [Carl Anders Düvel] [SPARK-9094] [PARENT] Increased io.dropwizard.metrics from 3.1.0 to 3.1.2 in order to get this fix https://github.com/dropwizard/metrics/commit/70559816f1fc3a0a0122b5263d5478ff07396991