In 178f976 you renamed your access transformer file from accesstransformer.cfg to simplyjetpacks_at.cfg, and as a result it is no longer loaded and applied. In order for FML to load the file it must be named accesstransformer.cfg.
You likely didn't notice this in dev because of the change made in build.gradle to reflect the filename change:

    accessTransformer = file('src/main/resources/META-INF/simplyjetpacks_at.cfg')

This, however, only applies to a dev environment.

Thank you !