Update Sinon to 1.8.1

Summary of changes between 1.7.1 and 1.8.1 from
http://sinonjs.org/Changelog.txt

== 1.8.1 / 2014-02-02

* Screwed up NPM release of 1.8.0, unable to replace it

== 1.8.0 / 2014-02-02

* Add clearImmediate mocking support to the timers API (Tim Perry)
* Mirror custom Date properties when faking time
* Improved Weinre support
* Update call properties even if exceptions are thrown (Tim Perry)
* Update call properties even if exceptions are thrown (Tim Perry)
* Reverse matching order for fake server (Gordon L. Hempton)
* Fix restoring globals on another frame fails on Firefox
  (Burak YiÃÅ¸it Kaya)
* Handle stubbing falsey properties (Tim Perry)
* Set returnValues correctly when the spied function is called as a
  constructor (Tim Perry)
* When creating a sandbox, do not overwrite existing properties when
  inject
  properties into an object (Sergio Cinos)
* Added withCredentials property to fake xhr (Geries)
* Refine behavior withArgs error message (Tim Fischbach)
* Auto-respond to successive requests made with a single XHR object
  (Jan SuchÃÂ½)
* Add the ability for mock to accept sinon.match matchers as expected
  arguments (Zcicala)
* Adding support for XMLHttpRequest.upload to FakeXMLHttpRequest
  (Benjamin Coe)
* Allow onCall to be combined with returns* and throwsException in stub
  behavior
  sequences (Tim Fischbach)
* Fixed deepEqual to detect properties on array objects
* Fixed problem with chained timers with delay=0 (Ian Lewis)
* Use formatio in place of buster-format (Devin Weaver)

== 1.7.3 / 2013-06-20

* Removed use of array forEach, breaks in older browsers (Martin Hansen)
* sinon.deepEqual(new Date(0), new Date()) returns true
  (G.Serebryanskyi)

== 1.7.2 / 2013-05-08

* Sinon 1.7 has split calls out to a separate file. This caused some
  problems, so 1.7.2 ships with spyCall as part of spy.js like it used
  to be.
Merge pull request #177 from Gazler/update-sinon

Update Sinon to 1.8.1