bump Go to 1.3

~~Requires #6539~~ merged
~~Requires #6684~~ merged
~~Requires #6913~~ merged
Requires #7170
Fixes #4983
Some cli integration tests are still failing with this branch.
@tiborvass @tianon just stumbled upon this comment; 

Even though it *redirects* to that location, if `http://golang.org/dl/go1.3.src.tar.gz` is the URL that is communicated on their website, using the URL that is redirected to feels like using an undocumented 'internal' API.

What if golang decides to use different download-servers, depending on location, or switches to another URL in the future?

No big problem, just my 2c