Add documentation about defaulting to env :dev about excoveralls

[parroty]

Thanks for the comment.

I got some failing tests while running coveralls due to the :dev database having extra content.

Could you describe this point a little more? How your mix file is configured?

[zmoshansky]

What I mean is that my dev database has all sorts of records in it. But, some of the tests phoenix generates fail when that data is present.

ex.) refute Repo.all(User) may fail if I have a user and my test expected an empty users table.

Regardless, it seems like the documentation should suggest to run MIX_ENV=test mix coveralls because otherwise tests could accidentally destroy the dev database which would be pretty unexpected.

[parroty]

Thanks for the information!