Dev mode is not respected when MIX_ENV is configured via environment variable

[joshuap]

Running the phoenix server like this, no errors should be reported because dev is an excluded env:

MIX_ENV=dev HONEYBADGER_API_KEY=secret mix phx.server

The result is that errors are reported, however. I think it's because the values in the excluded_envs configuration option are atoms, but when configured via an environment variable the env config option is a string.

I think that when reading the MIX_ENV environment variable we should convert it to an atom, since that seems to be what Mix does. Another option might be to get the value from Mix.env rather than the environment variable itself.

[joshuap]

Note: #96 should fix this (thanks @sorentwo!) so we just need to review/verify and merge.