Removed some constant allready defined warnings

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

lib/gitlab/theme.rb

 module Gitlab
   class Theme
-    BASIC  = 1
-    MARS   = 2
-    MODERN = 3
-    GRAY   = 4
-    COLOR  = 5
+    BASIC  = 1 unless const_defined?(:BASIC)
+    MARS   = 2 unless const_defined?(:MARS)
+    MODERN = 3 unless const_defined?(:MODERN)
+    GRAY   = 4 unless const_defined?(:GRAY)
+    COLOR  = 5 unless const_defined?(:COLOR)
 
     def self.css_class_by_id(id)
       themes = {

lib/gitlab/visibility_level.rb

@@ -5,9 +5,9 @@
 #
 module Gitlab
   module VisibilityLevel
-    PRIVATE  = 0
-    INTERNAL = 10
-    PUBLIC   = 20
+    PRIVATE  = 0 unless const_defined?(:PRIVATE)
+    INTERNAL = 10 unless const_defined?(:INTERNAL)
+    PUBLIC   = 20 unless const_defined?(:PUBLIC)
 
     class << self
       def values