bump Go to 1.3

~~Requires #6539~~ merged
Some cli integration tests are still failing with this branch.
Indeed, ignore me:
```console
$ HEAD -E 'http://golang.org/dl/go1.3.src.tar.gz'
HEAD http://golang.org/dl/go1.3.src.tar.gz
User-Agent: lwp-request/6.03 libwww-perl/6.05

302 Found
Connection: close
Date: Wed, 25 Jun 2014 20:08:30 GMT
Location: https://storage.googleapis.com/golang/go1.3.src.tar.gz
Server: Google Frontend
Content-Length: 0
Content-Type: text/html; charset=utf-8
Alternate-Protocol: 80:quic
Client-Date: Wed, 25 Jun 2014 20:08:30 GMT
Client-Peer: 74.125.193.141:80
Client-Response-Num: 1

...
```