Fixed domain wildcard issues

[kaspernj]

Closes #39

I have implemented two options to fix the wildcard domain issue:

1. The Rake task chooses a domain that isn't a wildcard
2. You can supply the domain you want used through an extra environment variable called HEROKU_APP_DOMAIN

[kaspernj]

I can confirm this fixes my own wildcard issues, that I mentioned in the original issue.

[jalada]

Thanks @kaspernj. For now can we split this in two and have one PR for fixing the wildcard issue, and another for introducing the HEROKU_APP_DOMAIN variable? Because the latter is part of a wider discussion around how we utilise ACME_DOMAIN (when provided).