set output precision to 16

proposed fix for #352 / #422

Tested with latest version of Bootstrap.  For the snippet posted by the OP, lessphp outputs:
```
.row-fluid [class*="span"] {
  display: block;
  width: 100%;
  min-height: 30px;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  float: left;
  margin-left: 2.127659574468085%;
  *margin-left: 2.074468085106383%;
}
```