Fix extension renaming

[TheDancingCode]

This replaces the somewhat makeshift solution for handling the extension renaming with the solution recommended by the gulp-util deprecation notes. It uses replace-ext, and should be compatible with gulp 3 and 4.

Fixes #91.

[lazd]

Thank you! Perhaps the gulp-util depreciation notes need to be updated accordingly...

[spencer-brown]

ah, this is much better. thanks!