Gulp intermediate rewrite, fixes 53, 54, and 55

Could use some code review here. Special note:

- sourcemap path change / regression in 2514808
- speed up Sass command: 63c396f. Cool/not cool?
- tests: how much to test, how little?

All comments welcome.
Wut, I had no idea it could be negative. Is this documented anywhere?

```
[00:05:32] gulp-ruby-sass: Missing dependencies. Ruby and Sass must be installed and available.
```

:)