Do not rely on `Mix.env`

[Damir Gainetdinov]

Since Mix is not(and should not) included to a release, executing this command inside a release:

Application.get_env(:airbrakex, :environment, Mix.env)

causes an exception like this:

** (exit) an exception was raised:
    ** (UndefinedFunctionError) undefined function Mix.env/0 (module Mix is not available)
        Mix.env()

You can either make environment param mandatory and remove default value(which is Mix.env) from get_env function call OR you can merge this PR #9. To be honest I don't know which option is better, probably the first one.

[Michał Kalbarczyk]

Making 'environment' param mandatory may brake your app after upgrade. #9 soultion is fine, and as you said it works on production without mix. Could you rename @env to @default_env ?

[Michał Kalbarczyk]

Merged! THanks!