use foreman-env gem for `.env` parsing

`Foreman::Env` has been extracted to a stand-alone gem with
`Dotenv` compatible changes.

Possible breaking changes:

* The string `\n` within double-quoted RHS values is expanded
  to a newline character. This matches current Foreman behavior,
  but is a change for how `Dotevn` treated '\n' inside double
  quotes.

* For YAML-like syntax, at least 1 space must follow the colon.
  A colon without trailing whitespace is invalid YAML.