fix for error in restore

Hi, thanks for this handy package!

I noticed an error when running on ruby 1.9.3 on ubuntu:
/usr/local/rvm/gems/ruby-1.9.3-p448/gems/es_dump_restore-0.0.4/lib/es_dump_restore/es_client.rb:10:in `initialize': wrong number of arguments (2 for 3) (ArgumentError)
	from /usr/local/rvm/gems/ruby-1.9.3-p448/gems/es_dump_restore-0.0.4/lib/es_dump_restore/app.rb:62:in `new'

This pull request is a simple fix to add a nil as the third argument to es_client.

cheers
Bob.