Add Searchkick::Relation#per as alias for per_page (#1736)

Fixes compatibility with api-pagination which expects `per` to be present.

Author: jaredmoody

lib/searchkick/relation.rb

@@ -409,6 +409,8 @@ def per_page(value = NO_DEFAULT_VALUE)
       end
     end
 
+    alias_method :per, :per_page
+
     def per_page!(value)
       check_loaded
       # TODO set limit?