Fix AR test suite error under Rubinius 2.0 #rbxday

[guilleiguaran]

I got this error running AR tests under Rubinius:

undefined method `to_time' on an instance of Time. (NoMethodError)

Backtrace:
  Kernel(Time)#to_time (method_missing) at kernel/delta/kernel.rb:79
     Time#<=> (compare_with_coercion) at /Users/guille/code/rails/activesupport
                                         /lib/active_support/core_ext/time
                                         /calculations.rb:310

The error is fixed including a missing require