Do not bump the size if we are less than the smallest value in the array

For example, a responsive site targetting mobiles which specifies `syze.sizes(480)` will get `lt480` when the size is less than 480px and `is480` when the size is greater than or equal to 480px. This is much more logical then specifying a dummy '0' entry to workaround the previous functionality where both would be marked as `is480`.