During the Go 1.5 development cycle, this package used to require the
standard library's encoding/xml package from Go 1.5 or later, but
https://go-review.googlesource.com/#/c/12772/ (which was submitted in
July 2015) made an internal fork of encoding/xml, as some
namespace related changes introduced in the Go 1.5 cycle were rolled
back in response to https://github.com/golang/go/issues/11841

Thus, this "go1.4" runtime check is no longer necessary. In the long
term, this package should use the standard library's version, and the
internal fork deleted, once https://github.com/golang/go/issues/13400 is
resolved. We could re-introduce a similar check at that time, although
it could be done at compile time (via a "go1.7" build tag) instead of at
runtime.

Change-Id: I18258443aa3d9b519e23106aedb189f25c35495d
Reviewed-on: https://go-review.googlesource.com/21634Reviewed-by: Andrew Gerrand <adg@golang.org>