looks like it is passed and accessed like a string

so unless I'm going insane, it should be a string.

fixes
```
rageshake.js:61 Warning: Failed prop type: The prop `onClick` is marked as required in `AccessibleButton`, but its value is `undefined`.
    in AccessibleButton (created by RoomHeader)
    in RoomHeader (created by RoomView)
    in div (created by RoomView)
    in RoomView (created by LoggedInView)
    in main (created by LoggedInView)
    in div (created by LoggedInView)
    in div (created by LoggedInView)
    in LoggedInView (created by MatrixChat)
    in MatrixChat
```

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/views/rooms/RoomPreviewBar.js

@@ -47,7 +47,7 @@ module.exports = React.createClass({
         // The alias that was used to access this room, if appropriate
         // If given, this will be how the room is referred to (eg.
         // in error messages).
-        roomAlias: React.PropTypes.object,
+        roomAlias: React.PropTypes.string,
     },
 
     getDefaultProps: function() {