fix: Run tests without depending on openssl (#685)

`reqwest` is only used for a crate example basic HTTP call:


https://github.com/rust-cli/config-rs/blob/1483f4cfc0169c857d5a7d551c7fab02eeb17f06/examples/async_source/main.rs#L61

When cloning the project repo and running tests, it'd fail without
OpenSSL devel package present on the system to build `reqwest`. I was
going to replace that with `rusttls` but it seems HTTPS isn't required
at all so opt-out of default features seems more prudent?

Author: epage