relx ignores -d (and --dev-mode) options

If I don't have `{dev_mode, true}.` in the `relx.config` I never get the 

    Dev mode enabled, release will be symlinked

message, no matter if I supply `--dev-mode` or not. If I put `{dev_mode, true}.` in the `relx.config` I always get it.

relx version 0.5.1