Fix inconsistency in elasticsearch config file template

With this change you can use the ruby 1.9 syntax for declaring
hash {a: "b"}.
Using this syntax would trigger this kind of error during chef
run :

Chef::Mixin::Template::TemplateError
------------------------------------
undefined method `url' for
{"url"=>"http://localhost:9200"}:Mash
Merge pull request #71 from gocardless/fix-es-template

Fix inconsistency in elasticsearch config file template