Removed some constant allready defined warnings

**What does this MR do?**
It makes sure that warnings of already defined constants are not shown anymore

**Why is this MR needed?**
Its a little annoying that you see constant warnings every time you run a command

The warnings it removes:
```
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:3: warning: already initialized constant Gitlab::Theme::BASIC
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:3: warning: previous definition of BASIC was here
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:4: warning: already initialized constant Gitlab::Theme::MARS
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:4: warning: previous definition of MARS was here
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:5: warning: already initialized constant Gitlab::Theme::MODERN
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:5: warning: previous definition of MODERN was here
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:6: warning: already initialized constant Gitlab::Theme::GRAY
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:6: warning: previous definition of GRAY was here
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:7: warning: already initialized constant Gitlab::Theme::COLOR
/Users/jeroen/Development/gitlabhq/lib/gitlab/theme.rb:7: warning: previous definition of COLOR was here
/Users/jeroen/Development/gitlabhq/lib/gitlab/visibility_level.rb:8: warning: already initialized constant Gitlab::VisibilityLevel::PRIVATE
/Users/jeroen/Development/gitlabhq/lib/gitlab/visibility_level.rb:8: warning: previous definition of PRIVATE was here
/Users/jeroen/Development/gitlabhq/lib/gitlab/visibility_level.rb:9: warning: already initialized constant Gitlab::VisibilityLevel::INTERNAL
/Users/jeroen/Development/gitlabhq/lib/gitlab/visibility_level.rb:9: warning: previous definition of INTERNAL was here
/Users/jeroen/Development/gitlabhq/lib/gitlab/visibility_level.rb:10: warning: already initialized constant Gitlab::VisibilityLevel::PUBLIC
/Users/jeroen/Development/gitlabhq/lib/gitlab/visibility_level.rb:10: warning: previous definition of PUBLIC was here

```