Update Placeholder to be compatible with older versions of React (#2491)

* Make `placeholder` element compatible with older versions of React

`React.Fragment` is only available in react >= 16.2, which does not
meet the dependency requirements specified by the package (react >=
0.14.0). Updates from `React.Fragment` -> `span` to provide coverage
for older versions of react.

* Update `slate-react` placeholder test case

packages/slate-react-placeholder/src/index.js

@@ -70,12 +70,12 @@ function SlateReactPlaceholder(options = {}) {
       }
 
       return (
-        <React.Fragment>
+        <span>
           <span contentEditable={false} style={style}>
             {placeholder}
           </span>
           {children}
-        </React.Fragment>
+        </span>
       )
     }
 

packages/slate-react/test/rendering/fixtures/placeholder/single-block-with-single-empty-text.js

@@ -21,6 +21,7 @@ export const output = `
   <div style="position:relative">
     <span>
       <span data-slate-leaf="true">
+        <span>
             <span contenteditable="false" style="pointer-events:none;display:inline-block;width:0;max-width:100%;white-space:nowrap;opacity:0.333">
                 placeholder text
             </span>
@@ -29,6 +30,7 @@ export const output = `
             </span>
         </span>
       </span>
+    </span>
   </div>
 </div>
 `