Regular HTTP is insecure, oeis.org supports HTTPS and it is actually
used in some other places in the codebase. This changes these final urls
to use HTTPS.

Change-Id: Ia46410a9c7ce67238a10cb6bfffaceca46112f58
Reviewed-on: https://go-review.googlesource.com/52072Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>