Fix interface conversion in task state metrics

[jacobjanco]

Fixes:

panic: interface conversion: prometheus.Collector is *prometheus.GaugeVec, not *main.settableCounterVec

goroutine 12 [running]:
main.newMasterCollector.func20(0xc4202c4120, 0x8abfe0, 0xc42000e080, 0x0, 0x0)
	/go/src/github.com/mesos/mesos_exporter/master.go:419 +0x6f7
main.(*metricCollector).Collect(0xc42009eb30, 0xc4200ba420)
	/go/src/github.com/mesos/mesos_exporter/common.go:295 +0x1e7
github.com/mesos/mesos_exporter/vendor/github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func2(0xc420024470, 0xc4200ba420, 0x8ac1e0, 0xc42009eb30)
	/go/src/github.com/mesos/mesos_exporter/vendor/github.com/prometheus/client_golang/prometheus/registry.go:383 +0x61
created by github.com/mesos/mesos_exporter/vendor/github.com/prometheus/client_golang/prometheus.(*Registry).Gather
	/go/src/github.com/mesos/mesos_exporter/vendor/github.com/prometheus/client_golang/prometheus/registry.go:381 +0x302

[jpeach]

Sorry @jacobjanco, I merged #86 since it was posted earlier