Log Message:
Update ruby-rspec-core to 2.13.1.

### 2.13.1 / 2013-03-12
[full changelog](http://github.com/rspec/rspec-core/compare/v2.13.0...v2.13.1)

Bug fixes

* Use hook classes as proxies rather than extending hook blocks to support
  lambdas for before/after/around hooks. (David Chelimsky)
* Fix regression in 2.13.0 that caused confusing behavior when overriding
  a named subject with an unnamed subject in an inner group and then
  referencing the outer group subject's name. The fix for this required
  us to disallow using `super` in a named subject (which is confusing,
  anyway -- named subjects create 2 methods, so which method on the
  parent example group are you `super`ing to?) but `super` in an unnamed
  subject continues to work (Myron Marston).
* Do not allow a referenced `let` or `subject` in `before(:all)` to cause
  other `let` declarations to leak across examples (Myron Marston).
* Work around odd ruby 1.9 bug with `String#match` that was triggered
  by passing it a regex from a `let` declaration. For more info, see
  http://bugs.ruby-lang.org/issues/8059 (Aaron Kromer).
* Add missing `require 'set'` to `base_text_formatter.rb` (Tom
  Anderson).

Deprecations

* Deprecate accessing `let` or `subject` declarations in `before(:all)`.
  These were not intended to be called in a `before(:all)` hook, as
  they exist to define state that is reset between each example, while
  `before(:all)` exists to define state that is shared across examples
  in an example group (Myron Marston).