Fix unit tests

As of commit `9a50ea77 "all: switch to gopkg.in"` unit tests are failing. First of all, `pool_test.go` imports the package by name `"github.com/go-pg/pg"` instead of canonical name for the package: `"gopkg.in/pg.v1"`. After fixing the import, `go test` produces the following output:
```
$ go test gopkg.in/pg.v1
# gopkg.in/pg.v1_test
src/gopkg.in/pg.v1/example_test.go:133: cannot use buf (type *bytes.Buffer) as type io.WriteCloser in function argument:
        *bytes.Buffer does not implement io.WriteCloser (missing Close method)
src/gopkg.in/pg.v1/main_test.go:352: cannot use buf (type *bytes.Buffer) as type io.WriteCloser in function argument:
        *bytes.Buffer does not implement io.WriteCloser (missing Close method)
src/gopkg.in/pg.v1/main_test.go:366: cannot use buf (type *bytes.Buffer) as type io.WriteCloser in function argument:
        *bytes.Buffer does not implement io.WriteCloser (missing Close method)
FAIL    gopkg.in/pg.v1 [build failed]
```

After applying the commits attached to this pull request, the tests pass again:
```
$ go test gopkg.in/pg.v1
ok      gopkg.in/pg.v1  10.719s
```