codereview: switch defaultcc to golang-codereviews
codereview: switch defaultcc to golang-codereviews

See this thread for background:
https://groups.google.com/forum/#!topic/golang-dev/xG7vPi21r8g

R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/40000044

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

VividCortex/goprotobuf,go-mirrors/goprotobuf