Avoid detecting SCSS files as CSS

Hey there.

I was having the same issues as #7 so i checked what was the problem and here is what i found:
The is_css method returned true even for synthax like `Packages/SCSS/SCSS.tmLanguage` (in my case) because of the endswith.
The fix i applyed is simple as you can see and works for me but i have never written any python before, so there may be a better way to do this.

Hope this helps ;)