scl: Fixed the elasticsearch SCL plugin

We can't break $(format-json) into multiple lines, as that produces a
syntax error. Changed the elasticsearch SCL plugin to have it on one
single line.

Reported-by: Peter Czanik <czanik@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #74 from algernon/h/elastic-scl-fix

scl: Fixed the elasticsearch SCL plugin