Update UIExplorer <Text /> example to respect ultralight font

Summary:Fixes #6735

On font sizes smaller than 20, iOS will swap SystemFont to SF version optimised for smaller sizes which does not have ultralight and thin. That results in ultralight and light example to be rendered with the same weight.

Bumping to `20` makes it render differently as per below screenshot https://cloud.githubusercontent.com/assets/2464966/14622260/0789e500-05c9-11e6-920a-8c948a5b79b4.png
Closes #7044

Differential Revision: D3212565

Pulled By: mkonicek

fb-gh-sync-id: 5ffad136c7f4126c271366640c89673f30a99218
fbshipit-source-id: 5ffad136c7f4126c271366640c89673f30a99218

Examples/UIExplorer/TextExample.ios.js

@@ -155,19 +155,19 @@ exports.examples = [
   render: function() {
     return (
       <View>
-        <Text style={{fontWeight: '100'}}>
+        <Text style={{fontSize: 20, fontWeight: '100'}}>
           Move fast and be ultralight
         </Text>
-        <Text style={{fontWeight: '200'}}>
+        <Text style={{fontSize: 20, fontWeight: '200'}}>
           Move fast and be light
         </Text>
-        <Text style={{fontWeight: 'normal'}}>
+        <Text style={{fontSize: 20, fontWeight: 'normal'}}>
           Move fast and be normal
         </Text>
-        <Text style={{fontWeight: 'bold'}}>
+        <Text style={{fontSize: 20, fontWeight: 'bold'}}>
           Move fast and be bold
         </Text>
-        <Text style={{fontWeight: '900'}}>
+        <Text style={{fontSize: 20, fontWeight: '900'}}>
           Move fast and be ultrabold
         </Text>
       </View>