Fixed less compiling issue

https://github.com/silviomoreto/bootstrap-select/blob/master/less/bootstrap-select.less#L229

This compiles into 
<pre>.bootstrap-select.show-menu-arrow.open > .btn {
  z-index: 1035 + 1;                                                                                                               
}
</pre>

which is not valid CSS syntax.

Math oparations should be wrapped with parantheses.