Remove hashes from commit numbers

**what is the change?:**
We had added hashes to some commit numbers, but ideally only do that
for PRs.

**why make this change?:**
Consistency - this is how github displays those types of links too. PRs
get a '#', and commits don't.

**test plan:**
Visual inspection

**issue:**
facebook#9398

CHANGELOG.md

@@ -13,10 +13,10 @@
 ## 15.5.4 (April 11, 2017)
 
 ### React Addons
-* **Critical Bugfix:** Update the version of `prop-types` to fix critical bug.  ([@gaearon](https://github.com/gaearon) in [#545c87f](https://github.com/facebook/react/commit/545c87fdc348f82eb0c3830bef715ed180785390))
+* **Critical Bugfix:** Update the version of `prop-types` to fix critical bug.  ([@gaearon](https://github.com/gaearon) in [545c87f](https://github.com/facebook/react/commit/545c87fdc348f82eb0c3830bef715ed180785390))
 
 ### React Test Renderer
-* Fix compatibility with Enzyme by exposing `batchedUpdates` on shallow renderer. ([@gaearon](https://github.com/gaearon) in [#9382](https://github.com/facebook/react/commit/69933e25c37cf5453a9ef132177241203ee8d2fd))
+* Fix compatibility with Enzyme by exposing `batchedUpdates` on shallow renderer. ([@gaearon](https://github.com/gaearon) in [9382](https://github.com/facebook/react/commit/69933e25c37cf5453a9ef132177241203ee8d2fd))
 
 
 ## 15.5.3 (April 7, 2017)
@@ -40,7 +40,7 @@
 **Note: this release has a critical issue and was deprecated. Please update to 15.5.4 or higher.**
 
 ### React
-* Fix erroneous PropTypes access warning. ([@acdlite](https://github.com/acdlite) in ([#ec97ebb](https://github.com/facebook/react/commit/ec97ebbe7f15b58ae2f1323df39d06f119873344))
+* Fix erroneous PropTypes access warning. ([@acdlite](https://github.com/acdlite) in ([ec97ebb](https://github.com/facebook/react/commit/ec97ebbe7f15b58ae2f1323df39d06f119873344))
 
 ## 15.5.0 (April 7, 2017)