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.
let use a `Set` here. Multiple gems could require the same vars.
That will avoid us having a messages like 
```
Must define the environment variable(s) FOO_API_KEY FOO_API_KEY FOO_API_KEY FOO_API_KEY
```