9.4 release post cover photo can make title text illegible

Originally mentioned on the MR: https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/6239#note_35655570

It looks like the issue was considered for smaller width sizes, the following rule makes the white gradient overlay more agressive below 480px:

@media (max-width: 480px)
release-posts.css:1
.blog.article .overlay {
    background: linear-gradient(to top, #fff 25%, transparent 55%);
}

This indeed helps legibility, but I think this rule should be applied at the 768px breakpoint instead to prevent the problem in all cases. The below two pictures are screenshots at 480px and 481px width. The right picture is the same as it looks like on my phone (a OnePlus One.)

/cc @marcia