Replace gulp-sass with gulp-ruby-sass

This changeset attempts to resolve #117 by replacing `gulp-sass` (`libsass`) with `gulp-ruby-sass` (Ruby Sass). The main tradeoffs are that Ruby Sass can be slower than `libsass` and requires the Ruby runtime, but Ruby Sass also appears to be more stable, doesn't require the Node bindings for `libsass`, and has all of the Sass features.

Arbitrary data points around build durations:

**gulp-ruby-sass**

```
./scripts/npm.sh run build-debug  1.29s user 0.27s system 30% cpu 5.042 total
./scripts/npm.sh run build-debug  1.28s user 0.27s system 31% cpu 4.957 total
./scripts/npm.sh run build-debug  1.29s user 0.27s system 31% cpu 4.955 total
```

**gulp-sass**

```
./scripts/npm.sh run build-debug  1.42s user 0.31s system 32% cpu 5.394 total
./scripts/npm.sh run build-debug  1.39s user 0.30s system 31% cpu 5.370 total
./scripts/npm.sh run build-debug  1.39s user 0.30s system 32% cpu 5.155 total
```

Sass test file:

```scss
$font-stack:    Helvetica, sans-serif;
$primary-color: #333;

body {
  font: 100% $font-stack;
  color: $primary-color;
}
```

I assume that as the amount of Sass piles up, the performance benefits of `gulp-sass` would breakthrough. Not sure though.