bump Go to 1.3

~~Requires #6539~~ merged
Some cli integration tests are still failing with this branch.
Shouldn't this be "http://golang.org/dl/go1.3.src.tar.gz" now, since the downloads have moved over to golang.org finally? (http://golang.org/dl/ and https://code.google.com/p/go/wiki/Downloads?tm=2)