Fixed an error was being raised as pointed at insin#12 even if one fo…

…llow the docs:

    Warning: Failed propType: Required prop `form` was not specified in `BootstrapForm`. Check the render method of `App`.

As far as I see the `required` of form is only used in order to guarantee it is used between a newforms.Form tag. But if you don`t pass it, an error is raised anyway:
   Uncaught TypeError: Cannot read property `__patchedByBootstrapForm` of undefined

So I removed the the "required" form property on propTypes and made a validation inside render.

src/index.jsx

@@ -184,6 +184,7 @@ var BootstrapForm = React.createClass({
   },
 
   propTypes: {
+    form: React.PropTypes.instanceOf(Form),
     spinner: React.PropTypes.string
   },