Also make tip a fast-finish allowed failure. That way, if CI fails on
tip due to a temporary issue with tip, it will not break build status.
However, it's still possible to see tip build status by looking at CI
details page.

Do not run go vet with Go 1.4 since it's not included in the standard
library, and it's no longer available in external standard library.

Adding the new BasicAuthTransport raises the minimum required version of
go to 1.4.