Fix for 1.4+ version of less.

Less introduced a new `--strict-math=off` flag. But apparently (less/less.js#1480) this doesn't apply to @media queries. So compiling Bootstrap 2.3.1 on less 1.5.1 will fail.

```
$ git clone git@github.com:320press/wordpress-bootstrap.git
$ cd wordpress-bootstrap/library
$ lessc less/responsive.less > css/responsive.css
$ lessc -v
lessc 1.5.1 (LESS Compiler) [JavaScript]
```

Can't submit fixes for Bootstrap any more, so might as well put this fix in here. It exists in Bootstrap 2.3.1 and 2.3.2.