getClasses function's filter works contrawise

[danielcsorba]

https://github.com/Bttstrp/bootstrap-switch/blob/19275e72733674f67e617139b0e41c93db1aeaf0/src/js/bootstrap-switch.js#L15

Result: This currently leaving only the null values in the array. Causing invalid class names and preventing class based options to work.

Fix: It should be negated !== instead of ==.