Build against Go 1.3 but don't fail builds if it has issues

Go 1.3 has been officially released: http://blog.golang.org/go1.3

We should start building against this now to make sure that we're
keeping up-to-date with releases and to make life easier for us if we
do decide to upgrade our deployable builds to use Go 1.3.

Setting `fast_finish` to `true` so that we don't have to wait on all
components of the build matrix to complete before we get
notifications. Builds for 1.2 should be enough to trigger a status
update for the time being.
Merge pull request #18 from alphagov/test-go-1.3-but-dont-fail-build-on-failures

Build against Go 1.3 but don't fail builds if it has issues