fixed checking for easing css class

NOTE: I have never used your plugin at all, nor have I tested this pull request!
consider this a proposal of how I would attempt to solve the issue (issue discussed here: https://github.com/charliegeiger89/Scrolld.js/commit/0a10b6ed39bbd3859b551e359e20a32483bd75e9#commitcomment-4553480)

please thoroughly test this code before merging.