Default to a clean build when build_before is used

In middleman 3.1, middleman-core now (correctly) [defaults to running `--clean` builds](https://github.com/middleman/middleman/blob/master/CHANGELOG.md#310-highlights), but middleman-deploy does not run clean builds when `build_before` is turned on.  

middleman-deploy should match the defaults of middleman-core and ensure clean builds before a deploy.

Related to #24