Use FlxBaseTilemap<Dynamic> instead of FlxTilemap

Since the changes in FlxTilemap, it makes more sense to use FlxBaseTilemap when checking for collisions. This will allow any FlxBaseTilemap sub classes to use these methods.
Merge pull request #1107 from Tiago-Ling/patch-1

Use FlxBaseTilemap<Dynamic> instead of FlxTilemap