Travis must always test current branch, not gopkg.in/pg.v1

It appears that until now travis tested `gopkg.in/pg.v1` package --- that is, whichever commit was tagged as `v1`. So `master` branch itself and pull requests went untested.

After changing travis config, it turns out that "Fix setting zero values on NULL" actually doesn't work: https://travis-ci.org/go-pg/pg/jobs/27562268