Fix demo GIF

I originally added this to the gh-pages branch so that the GIF wouldn't
bloat the repo or the package that people install. However, I didn't
realize that we have a script that blows all of those files away
whenever we publish, so it quickly disappeared. We've decided that repo
bloat is okay, and we can .npmignore this file for the sake of
installers.

While I was at it, I sorted some things alphabetically in the .npmignore
file.

.npmignore

@@ -34,6 +34,7 @@ node_modules
 
 .storybook
 examples
+public
+react-dates-demo.gif
 stories
 test
-public

README.md

@@ -10,7 +10,7 @@
 
 An accessible, easily internationalizable, mobile-friendly datepicker library for the web.
 
-![react-dates in action](https://raw.githubusercontent.com/airbnb/react-dates/gh-pages/images/react-dates.gif)
+![react-dates in action](https://raw.githubusercontent.com/airbnb/react-dates/master/images/react-dates-demo.gif)
 
 ## Live Playground