Update travis config

Update dependencies

Fix unused variable warnings; Update styling

Update exprintf dependency

There are no Rails where we're going

Update README with nullify example

Guard all the things

Add test cases to ensure ENV is unset properly

Add ability to unset ENV variables when specified

Ensure env-specific nulls override default config

Bump version

Specify that env-specific configs use Mix.env

Update documentation for env-specific config

Add ability to include env-specific configuration

Use yamler in favor over yamerl