bundler: do not add a ruby name prefix

Otherwise bundler replaces ruby:

% nix-env -iA nixpkgs.ruby
installing ‘ruby-1.9.3-p547’
% nix-env -iA nixpkgs.bundler
replacing old ‘ruby-1.9.3-p547’
installing ‘ruby-1.9.3-p547-bundler-1.9.2’