Update issue links to point to sinonjs/sinon

We transferred ownership a while back, and shouldn't make people wait a
full roundtrip to get a redirect from GitHub.

CONTRIBUTING.md

@@ -7,7 +7,7 @@ There are several ways of contributing to Sinon.JS
   at [the Sinon.JS website](http://sinonjs.org). [Documentation issues](https://github.com/sinonjs/sinon/issues?q=is%3Aopen+is%3Aissue+label%3ADocumentation).
 * Help someone understand and use Sinon.JS on [Stack Overflow](https://stackoverflow.com/questions/tagged/sinon)
 * Report an issue, please read instructions below
-* Help with triaging the [issues](http://github.com/cjohansen/Sinon.JS/issues). The clearer they are, the more likely they are to be fixed soon.
+* Help with triaging the [issues](https://github.com/sinonjs/sinon/issues). The clearer they are, the more likely they are to be fixed soon.
 * Contribute to the code base.
 
 ## Reporting an issue
@@ -28,9 +28,9 @@ See [our issue template](https://github.com/sinonjs/sinon/blob/master/.github/)
 
 ## Contributing to the code base
 
-Pick [an issue](http://github.com/cjohansen/Sinon.JS/issues) to fix, or pitch
+Pick [an issue](https://github.com/sinonjs/sinon/issues) to fix, or pitch
 new features. To avoid wasting your time, please ask for feedback on feature
-suggestions with [an issue](http://github.com/cjohansen/Sinon.JS/issues/new).
+suggestions with [an issue](https://github.com/sinonjs/sinon/issues/new).
 
 ### Making a pull request