Allow lookup service to fully opt-out of https

If you have an address geocoding service that you want to use HTTPS with
and then attempt to use an ip_geocoder that does not support HTTPS (e.g.
Telize) then the ip_geocoder lookups will fail because
Geocoder::Lookups::Base#make_api_request turns on opts[:use_ssl] based
on that single configuration value.

Rather than allowing for per-provider HTTPS configuration I have just
refactored Geocoder::Lookups::Base call a private `use_ssl?` method
which can be overridden by individual lookup provider implementations.
The default implementation of `use_ssl?` simply returns the value from
the config thus preserving the existing behavior but this allows
subclasses, like Telize, to override `use_ssl?` completely to return
false and thus fully opt-out of HTTPS regardless of what the
configuration was.
Merge pull request #683 from CheckMateIO/lookups-opt-out-of-https

Allow lookup service to fully opt-out of https