Rescue Etc.getgrgid so that the fallback code path is possible

[sodabrew]

There's a fallback code path if the group name cannot be found, but it can't run if Etc.getgrgid raises an exception, e.g. ArgumentError: can't find group for 285

      def current_group
        group_obj = Etc.getgrgid
        make_valid_group_name(group_obj ? group_obj.name : "dummygroup")
      end

[larskanis]

On which operating system does getgrpid raise an exception?

[sodabrew]

On OS X if the group doesn't have a name set, (maybe because it's a NetInfo group?)

[larskanis]

Thank you @sodabrew ! I just released rake-compiler-dock-0.5.2 with your change.