Fix respond-range mixin when lap breakpoint is used

Closes #125 

This: 
```
@include respond-range(lap) {
     .foo {background: red;}
}
```

now generates

```
@media (min-width: 40.0625em) and (max-width: 75em) {
  .foo {
    background: red; } }
```

instead of

```
@media (min-width: 40.0625em) and (max-width: 56.3125em) {
  .foo {
    background: red; } }
```