JSON.dump backup_destination instead of YAML.dump(...).sub(...)

YAML.dump is slightly inconsistent across ruby/yaml versions so the
substitution doesn't always work. Just JSON.dump in the one place we use
it since JSON is a subset of YAML

[#96930436](https://www.pivotaltracker.com/story/show/96930436)

Signed-off-by: Colin Obyrne <cobyrne@pivotal.io>

release/jobs/director/templates/director.yml.erb.erb

@@ -1,10 +1,3 @@
-<%
-  require "yaml"
-
-  def dump_yaml(name, options)
-    YAML.dump(name => options).sub(/\A---\n/, '')
-  end
-%>
 ---
 name: <%= p('director.name') %>
 port: <%= p('director.backend_port') %>
@@ -219,7 +212,7 @@ compiled_package_cache:
 <% end %>
 
 <% if_p('director.backup_destination') do |backup_destination| %>
-<%= dump_yaml('backup_destination', backup_destination) %>
+backup_destination: <%= JSON.dump(backup_destination) %>
 <% end %>
 
 cloud: