Update statsd reporting README instructions

[jessicayuen]

• Fix invalid variable reference, s/r/reporter
• Remove references to legacy statsd client

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[coveralls]

Coverage remained the same at 93.545% when pulling 7c87b71 on jessicayuen:readme-update into 798e659 on uber-go:master.

[coveralls]

Coverage remained the same at 93.545% when pulling 7c87b71 on jessicayuen:readme-update into 798e659 on uber-go:master.