Keep the value type for YAML variables

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8088#note_20235080

lib/ci/gitlab_ci_yaml_processor.rb

@@ -118,7 +118,7 @@ module Ci
         .merge(job_variables(name))
 
       variables.map do |key, value|
-        { key: key.to_s, value: value.to_s, public: true }
+        { key: key.to_s, value: value, public: true }
       end
     end
 

lib/gitlab/serialize/ci/variables.rb

@@ -14,7 +14,6 @@ module Gitlab
 
           object.map do |variable|
             variable[:key] = variable[:key].to_s
-            variable[:value] = variable[:value].to_s
             variable
           end
         end