Install Ruby in the backup cookbook

Also switch to the 5.0 beta version, it works with modern Rubies

site-cookbooks/backup/metadata.rb

@@ -8,3 +8,5 @@ name "backup"
 
 depends 'logrotate'
 depends 'mysql'
+depends 'postgresql'
+depends 'build-essential'

site-cookbooks/backup/templates/default/config.rb.erb

@@ -1,7 +1,7 @@
 # encoding: utf-8
 
 ##
-# Backup v4.x Configuration
+# Backup v5.x Configuration
 #
 # Documentation: http://backup.github.io/backup
 # Issue Tracker: https://github.com/backup/backup/issues
@@ -56,8 +56,8 @@ end
 
 <%- if node["backup"]["postgresql"] -%>
 Database::PostgreSQL.defaults do |db|
-  db.username           = "<%= node["backup"]["postgresql"]["username"] %>"
-  db.password           = "<%= node["backup"]["postgresql"]["password"] %>"
+  db.username           = "postgres"
+  db.password           = "<%= node['postgresql']['password']['postgres'] %>"
   db.host               = "<%= node["backup"]["postgresql"]["host"] %>"
   db.port               = "<%= node["backup"]["postgresql"]["port"] %>"
   # db.socket             = "/var/run/postgresql/.s.PGSQL.5432"