Feature/libsass

Let's retain your use of "Ruby" and "LibSass" as the CLI prompts. However, convert the `sassComp` variable to lowercase like the `jsPre` variable is doing on [line 103](https://github.com/centresource/generator-playbook/pull/49/files#diff-2018087f584c4398b5c3a23fc0e5f9dbR103), then update all the Ruby/LibSass conditionals to use lowercase text as well (e.g. – [here](https://github.com/pocketbird/generator-playbook/blob/feature/libsass/app/templates/_Gruntfile.js#L116)).