minor optimization

* Array(list) covers all necessary cases

    $ irb
    2.0.0p0 :001 > Array(nil)
     => []
    2.0.0p0 :002 > Array(:foo)
     => [:foo]
    2.0.0p0 :003 > Array([:foo,:bar])
     => [:foo, :bar]

* Array(list) is considerably faster than list.flatten.compact

    $ time ruby -e "100000.times { [nil].flatten.compact; [:foo].flatten.compact; [[:foo, :bar]].flatten.compact }"
    real 0m0.465s
    user 0m0.461s
    sys  0m0.003s
    $ time ruby -e "100000.times { Array(nil); Array(:foo); Array([:foo, :bar]) }"
    real 0m0.117s
    user 0m0.113s
    sys  0m0.003s

lib/rspec/core/configuration.rb

@@ -72,7 +72,7 @@ def self.add_setting(name, opts={})
           define_reader name
           define_predicate_for name
         end
-        [opts[:alias_with]].flatten.compact.each do |alias_name|
+        Array(opts[:alias_with]).each do |alias_name|
           define_aliases(name, alias_name)
         end
       end