[MODEL] Updated the dependency on "elasticsearch" to 5.x

The 5.x versions should depend on the corresponding version of the Ruby client.

Related: #702

Author: karmi

elasticsearch-model/elasticsearch-model.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |s|
 
   s.required_ruby_version = ">= 1.9.3"
 
-  s.add_dependency "elasticsearch",       '> 1'
+  s.add_dependency "elasticsearch",       '~> 5'
   s.add_dependency "activesupport",       '> 3'
   s.add_dependency "hashie"