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/36380046

lib/codereview/codereview.cfg

-defaultcc: golang-dev@googlegroups.com
+defaultcc: golang-codereviews@googlegroups.com