Updates go-statsd-client to 2.0.2.

[cpu]

This commit updates the cactus go-statsd-client dependency to the 2.0.2 release tag.

Note: the most recent release is 3.1.0 but the introduction of substatter support in 3.0.0 changes the Statter interface we rely on in the boulder metrics package. We should consider follow-up work to refactor our metrics code to be compatible with the 3.x releases.

Per CONTRIBUTING.md I verified the unit tests pass:

daniel@XXXXXXX:~/go/src/github.com/cactus/go-statsd-client$ git show -s
commit 26a31053503dcd96f51363a90eb03e0b32406e26
Author: elij <elij@wundrd.net>
Date:   Fri Oct 16 14:51:29 2015 -0700

    update changelog for 2.0.2

daniel@XXXXXXX:~/go/src/github.com/cactus/go-statsd-client$ go test -v ./...
=== RUN   TestBufferedClientFlushSize
--- PASS: TestBufferedClientFlushSize (0.00s)
=== RUN   TestBufferedClientFlushTime
--- PASS: TestBufferedClientFlushTime (0.02s)
=== RUN   TestBufferedClientBigPacket
--- PASS: TestBufferedClientBigPacket (0.01s)
=== RUN   TestFlushOnClose
--- PASS: TestFlushOnClose (0.00s)
=== RUN   TestClient
--- PASS: TestClient (0.00s)
=== RUN   TestNilClient
--- PASS: TestNilClient (0.10s)
=== RUN   TestNoopClient
--- PASS: TestNoopClient (0.10s)
PASS
ok    github.com/cactus/go-statsd-client/statsd 0.242s
?     github.com/cactus/go-statsd-client/statsd/test-client [no test files]

Relates to #2315.