move the conditional definition of Float#round down

Rails 3 cherry-picks as little as possible from Active Support. In particular
the framework requires the files where extensions are exactly defined, rather
than convenience entry points like float.rb and friends. Because of that, it
is better to define conditionally rather than require conditionally.

activesupport/lib/active_support/core_ext/float.rb

@@ -1 +1 @@
-require 'active_support/core_ext/float/rounding' if RUBY_VERSION < '1.9'
+require 'active_support/core_ext/float/rounding'

activesupport/lib/active_support/core_ext/float/rounding.rb

@@ -16,4 +16,4 @@ class Float
       precisionless_round
     end
   end
-end
+end if RUBY_VERSION < '1.9'