I am fairly new to archlinux. Postgresql 11 is fairly new too. Here are the steps to install that version.

# get the lattest postgres in the repos
# avoid checking for sha fingerprint, because not the same version
yaourt --m-arg "--skipchecksums --skippgpcheck" -Sb postgresql
# 1. edit the PKGBUILD file, change to 11.0 version
# 2. comment the check() part
# 3. after building and installing, change to 11 the PGMAJORVERSION within:
sudo vim /usr/bin/postgresql-check-db-dir
# make the link for psql command working
sudo ln -s /usr/lib/libreadline.so.7 /usr/lib/libreadline.so.6
# install that old version to make posgres command work
yaourt -S icu54

That's all. By the way, I do not have any idea right now on how to upgrade minor versions in the future, but I will employ myself to update this post.