fs-p is deprecated

[balassy]

When running npm install in a project that depends on serverless-plugin-typescript, npm throws the following warning:

npm WARN deprecated fs-p@2.0.0: Use fs-extra@4.0.0+ instead

Thank you for creating and updating this project!

[schickling]

Would you mind creating a PR replacing the dep?

[balassy]

Sure, here it is: #36.

Note that although I checked that all jest tests are passing, I could not thoroughly test the whole package.