[COOK-2383] Use postgresql 9.0 on Amazon Linux

Signed-off-by: Seth Vargo <sethvargo@gmail.com>
[COOK-3553] Remove apt-get update

Signed-off-by: Seth Vargo <sethvargo@gmail.com>
Support new unix_socket_directories config name in 9.3
Fix omnibus pg gem installation for Debian/Ubuntu, also install server packages before trying to install the pg gem, otherwise pg_config is never found.
Add a note about unix_socket_directories in README