In support-legacy-browser(), threshold should not override browser minimum versions

By default, `support-legacy-browser(ie, "7")` returns `true` because IE 7 browser usage is above the `$critical-usage-threshold`. That’s by design. Works for me!

However, if I specify `$browser-minimum-versions: ('ie': '8')`,  then `support-legacy-browser(ie, "7")` should return `false`. We are explicitly saying we want IE8 and higher only. Unfortunately, `support-legacy-browser()` still returns `true`.

Take this file for example: http://sassmeister.com/gist/e0aa37ca8d480d078ca5
```scss
.test {
  @if support-legacy-browser(ie, "7") {
    content: 'ie7';
  }
  @else {
    content: 'ie8 and higher';
  }
}
```

No matter what we set `$browser-minimum-versions` to, the 'ie7' line is always output.

There's actually a few lines of CSS in compass-core that are broken because of this bug.