main-bower-files is not suitable for the current bower.json spec.

[htanjo]

We used main-bower-files to copy assets—such as images, fonts, etc.—from bower_components.
But Bower changed the "main" field specification and now main-bower-files is not very suitable.

"main" - bower.json spec
https://github.com/bower/spec/blob/master/json.md#main

Actually Bootstrap removed font files from "main" to follow the current spec, this means main-bower-files is not compatible with Bootstrap now.

We should respect the official specification and remove main-bower-files from the generator-rff.