Prevent making useless symbols.

Groups are concatenated with ':' in Bundler::Settings#without=,
and splited by ':' in Bundler::Settings#without. So replace ' '
with ':' to prevent making useless symbols and other objects.

lib/bundler/cli.rb

@@ -161,9 +161,7 @@ def check
     def install
       opts = options.dup
       if opts[:without]
-        opts[:without].map!{|g| g.split(" ") }
-        opts[:without].flatten!
-        opts[:without].map!{|g| g.to_sym }
+        opts[:without] = opts[:without].map{|g| g.tr(' ', ':') }
       end
 
       # Can't use Bundler.settings for this because settings needs gemfile.dirname