When looking for related posts, Jekyll was indexing `Jekyll::Post`
objects, but finding related posts based on `Jekyll::Post#content`. This
caused two problems:
1. Ruby 2.2 will warn on == if <=> throws an exception (and future Ruby
versions will surface that exception). Because `String`s can't be
compared with `Jekyll::Post`s, this warning was appearing all the time
while searching for related posts.
2. LSI won't return a post itself when searching for related posts. But
LSI could never tell that we were searching on a post, since Jekyll
passed post content, not a post object. With this fix, we can remove the
`- [post]` from `Jekyll::RelatedPosts#find_related`.
This is a more accurate fix for #3484.
Since we don't actually use the index in getting the related posts from
the tests there's no need to build an index, which can take a long time
if the ruby bindings for the GSL library are not installed.
