Update for 2.4 branch.

* Set the module depenencies as f27.
* Fix ruby's commit reference to guarantee to provide the version 2.4.

ruby.yaml

@@ -20,10 +20,10 @@ data:
         tracker: https://bugs.ruby-lang.org/
     dependencies:
         buildrequires:
-            platform: master
-            autotools: master
+            platform: f27
+            autotools: f27
         requires:
-            platform: master
+            platform: f27
     components:
         # SRPMs
         rpms:
@@ -31,7 +31,8 @@ data:
                 # From ruby.spec Summary
                 rationale: An interpreter of object-oriented scripting language
                 buildorder: 101
-                ref: master
+                # Set a reference on master to guarantee the Ruby version 2.4.
+                ref: ad05a88
             rubygem-bundler:
                 # From rubygem-bundler.spec Summary
                 rationale: >-