Fix bootstrap3 select-width-item-border incompatible value

[nbluis]

The propose of the change is to solve a bug when we try to import the dist/selectize.bootstrap3.css using a CSS compiler like SASS.

The compiler throw an error of incompatible type in the calc function because we are trying to subtract a pixel value from a non defined value 0.

Error: 5px and 0 are incompatible.
  ╷
2 │       6px - 1px - 0
  │       ^^^^^^^^^^^^^
  ╵
  @selectize/selectize/dist/css/selectize.bootstrap3.css 2:7  @import

With this change, the result of this expressions will be like 6px - 1px - 0px instead of
6px - 1px - 0 making it compile correctly.