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://golang.org/cl/39440047

codereview.cfg

-defaultcc: golang-dev@googlegroups.com
+defaultcc: golang-codereviews@googlegroups.com
 contributors: http://go.googlecode.com/hg/CONTRIBUTORS