Allow [Symbol] when loading YAML

lib/ci/gitlab_ci_yaml_processor.rb

@@ -10,7 +10,7 @@ module Ci
     attr_reader :before_script, :image, :services, :variables, :path, :cache
 
     def initialize(config, path = nil)
-      @config = YAML.safe_load(config)
+      @config = YAML.safe_load(config, [Symbol])
       @path = path
 
       unless @config.is_a? Hash
@@ -250,4 +250,4 @@ module Ci
       end
     end
   end
-end
+end
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