Updating dependencies

• minimal required version is node 6 now
• do not depend on Promise polyfill, rely on native.

v2.0

Breaking changes:

• Fix #12 allows to import *.css files if they were matched by glob. If you don't want to import CSS-files, please be sure that your glob expression doesn't match them.
• Node.js bellow v4 are not supported now.

Recursive imports support

1.1.0

provide addtional testcases due to GH-1