remove `Dependency` section

This module doesn’t require libjpeg (or libjpeg-turbo) because it
downloads the binary files of jpeg-recompress directly, instead of
building from source.
Actually, the test on CI passes without installing mozjpeg.
(https://travis-ci.org/1000ch/node-jpeg-recompress-bin)
invoke callback with an error as its argument
Merge pull request #7 from shinnn/master

Remove `Dependency` section form README