Make dialyxir and ex_doc runtime: false

[fhunleth]

Luckily neither was available for :prod builds, so the dependencies would propogate to hex.pm. However, there was some confusion on other projects when they got built into the runtime issue and at least one person ran into wx being pulled in by dialyxir. This changes avoids the surprise by forcing those deps to runtime: false. It's also important that their :only settings doesn't let them to propogate into :prod builds, but that wasn't a problem here.

[adkron]

Thanks, @fhunleth. I will push a new release today.

[fhunleth]

Thanks!