Require environment variables to be defined at startup

Added the config option required_env_vars. This option allows the developer to specify a series of environment variables that must be defined at started, otherwise an exception is thrown.

```
  # e.g.
  config.required_env_vars = ['rollbar_api_key', 'github_api_key']
```
If either the ENV['rollbar_api_key'] or ENV['github_api_key'] environmental variables aren't defined, an exception is thrown and the server fails to startup.

We've had several instances where our ops team forgot to export an API key on the server and it wasn't apparent until later.  

Especially useful for systems like Heroku where reliance on environment variables is prevalent for Rails apps.
Currently when someone uses symbols (just like with the log_tags-config right below), it will raise (no matter the contents of ENV) but it won't be clear from the error why (e.g. assigning [:A] will err with message "Must define the environment variable(s) A").
To anticipate this, either the lookup could be made more flexible (`ENV[key.to_s]`) (or the error message more clear ("Must define the environment variable(s) :A")).

Second: as `ENV['SECRET_KEY_BASE']` is contained in the default `secrets.yml`, it might be a good candidate to either include in the example or even uncomment this line and just set it?

Third (nitpick): upcasing ENV-keys seems a bit odd IMO, in that it adds more surprise than value.
Besides that, while not common/recommended, users might actually have lowercase ENV-keys.