Revert "Remove the lowest memory requirement of 512MB."

This reverts commit 5cf6d5949d6c776e24d3bd5c0b417000f1efc57a.
System with 512MB of RAM can be used but swap size must be increased.
Merge branch 'reintroduce_minimum_gitlab_resources' into 'master'

Reintroduce minimum gitlab system requirement

As per https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1249#note_29946

See merge request !1286