Standalone metrics-healthchecks use crashes, due to Clock dependency

[tingley]

Since version 4.1.0, it's been impossible to include the metrics-healthchecks library as a standalone dependency, due to an unresolved class error for com.codahale.metrics.Clock:

java.lang.NoClassDefFoundError: com/codahale/metrics/Clock
	at com.codahale.metrics.health.HealthCheck.clock(HealthCheck.java:383) ~[metrics-healthchecks-4.2.0.jar:4.2.0]
	at com.codahale.metrics.health.HealthCheck.execute(HealthCheck.java:371) ~[metrics-healthchecks-4.2.0.jar:4.2.0]
	at com.codahale.metrics.health.HealthCheckRegistry.runHealthChecks(HealthCheckRegistry.java:184) ~[metrics-healthchecks-4.2.0.jar:4.2.0]
	at com.codahale.metrics.health.HealthCheckRegistry.runHealthChecks(HealthCheckRegistry.java:169) ~[metrics-healthchecks-4.2.0.jar:4.2.0]
      [... calling code ...]

I am fairly sure this is a result of #1372.

The simplest fix would simply be to list metrics-core as a dependency.

[tingley]

@joschi Thanks for the quick fix!