Fix: adding a project to a workspace is broken on 0.17.0

Since version 0.17.0, workspace.rb uses new FileReference system instead of string paths (#150), but there was a bug when executing “<<“ method:

    /Library/Ruby/Gems/2.0.0/gems/xcodeproj-0.17.0/lib/xcodeproj/workspace.rb:75:in `<<': 
         undefined method `path' for "MyProject.xcodeproj":String (NoMethodError)

Method “<<“ was missing a conversion from the input string path to a FileReference instance.