Only update simplemde value if the value has changed

Fixes an issue where react renders because of a change that doesn't involve simpleMDE, and the cursor in simpleMDE would go to the top. Traced it down to this, we should only be setting the `simplemde.value` if the next value does not equal the current value.

Author: Grant Ammons

src/index.js

@@ -33,7 +33,7 @@ module.exports = React.createClass({
   },
 
   componentWillReceiveProps: function(nextProps) {
-    if (!this.state.keyChange) {
+    if (!this.state.keyChange && (nextProps.value !== this.simplemde.value())) {
       this.simplemde.value(nextProps.value)
     }