fix sourceRoot

My project structure:
coffee
--folder1
----0.coffee
----folder2
------1.coffee
------2.coffee

js
--folder1
----0.js
----folder2
------1.js
------2.js

if I use plugin with options ```sourcemaps.write('./', {includeContent: false, sourceRoot: 'coffee' }``` I get error in browser while debugging "coffee/folder1/folder2/1.coffee not found", chrome looking for source in root of coffee folder.