Badge images in README are broken

Summary

This is a relatively very minor inconvenience, but a couple of broken images at the very start of our README isn't ideal either :)

Steps to reproduce

View README at https://gitlab.com/BuildStream/buildstream/blob/master/README.rst.

What is the current bug behavior?

Image sources for the smapshot and release badges point to URLs that return 404.

Possible fixes

One simple fix would be to revert !876 (merged). Seems like the "old" links are still correct. However, I personally don't quite understand the split between the two websites so wanted to get a second opinion.