Remove gists and fix iOS instructions

Summary:
Remove gists in code blocks. This also made it easier to fix the iOS sample app instructions in the getting started.

>Note: I also needed to update my gems as I was having problems running `bundle install`. The errors were around installing json v1.8.3. It was resolved by creating a new Gemfile.lock.

1. Build website and check changed pages (see attached Getting Started for an example)
2. Also check how it would look for mobile site (see attached index page)

Verified that the code snippets matched previous snippets and styling acceptable.

![yoga_gs](https://cloud.githubusercontent.com/assets/691109/24681238/1601fcc8-1949-11e7-8caa-5ac78a4c9a6b.png)
![yoga_index_mobile](https://cloud.githubusercontent.com/assets/691109/24681248/1ee3ea86-1949-11e7-9677-83056f93e385.png)
Closes https://github.com/facebook/yoga/pull/500

Differential Revision: D4834356

Pulled By: emilsjolander

fbshipit-source-id: f47dca4b7518822b195f0bd5076fbf852904372b

docs/_docs/learn-more.md

@@ -15,7 +15,23 @@ Yoga aims to be compatible with Flexbox according to the [w3 specification](http
 
 Yoga has chosen to change the default values of some properties to better fit mobile layout use cases. The following CSS block describes the differences in default values from the [Flexbox w3 specification](https://www.w3.org/TR/css3-flexbox).
 
-<script src="https://gist.github.com/emilsjolander/f9b3981cab44c51afa9ac446b8fdb60c.js"></script>
+```css
+div {
+  box-sizing: border-box;
+  position: relative;
+
+  display: flex;
+  flex-direction: column;
+  align-items: stretch;
+  flex-shrink: 0;
+  align-content: flex-start;
+
+  border-width: 0px;
+  margin: 0px;
+  padding: 0px;
+  min-width: 0px;
+}
+```
 
 We have set up a [JSFiddle](https://jsfiddle.net/emilsjolander/jckmwztt/) so you can see these default values in action.