Update Sinon to 1.8.1

[Gazler]

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 YiÄŸit 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 Suchý)
• 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.