Update selectize.less

Fixes issue when trying to compile with less 1.7.0

>> ArgumentError: error evaluating function `round`: argument must be a number in bower_components/selectize-js/dist/less/selectize.less on line 266, column 15:
>> 265          right: @selectize-arrow-offset;
>> 266          margin-top: round(-(@selectize-arrow-size / 2));
>> 267          width: 0;