droppable attribute is not respected.

[akshay-kr]

Docs mentions that droppable attribute defines Whether the column is a valid drop target..
I tried setting it to false for a column but still it gets displaced by dropping another column on it.

I think we should have a check for droppable attribute after below line,
https://github.com/offirgolan/ember-light-table/blob/master/addon/mixins/draggable-column.js#L116

if(targetColumn.droppable){ 
  //then proceed
}

Created a PR for this #496