bump go1.4rc1 tag

Now that the build and builders are fixed, we're good to go.

LGTM=dsymonds
R=rsc, dsymonds
CC=golang-codereviews
https://golang.org/cl/177900043

.hgtags

@@ -136,4 +136,4 @@ f8b50ad4cac4d4c4ecf48324b4f512f65e82cc1c go1.3beta1
 f44017549ff9c3cc5eef74ebe7276cd0dfc066b6 go1.3.3
 f44017549ff9c3cc5eef74ebe7276cd0dfc066b6 release
 1fdfd7dfaedb1b7702141617e621ab7328a236a1 go1.4beta1
-122dcdd033354e3208878c428783906ca7588964 go1.4rc1
+bffdd0cae380ce1ccf3e98ed6b6cd53fece7ba72 go1.4rc1