avoid overriding `use_ssl?` or `protocol` directly

Tests were failing like this:
  <Geocoder::Result::Nominatim> expected but was
  <"Geocoder::Result::Nominatim">

Treat Bing overload responses as invalid

Silence warnings using silence_warnings blocks

Support maxminddb gem v0.1.7

This reverts commit 4ba15c82, reversing
changes made to c740bbb7.

Conflicts:
	lib/geocoder/lookups/mapquest.rb
	test/services_test.rb

This was preventing lookups from forcing a particular protocol.

Don't include test files in the exported gem

To run a single file, use:

  ruby -I test <filename>

Update background job example

* Per discussion from https://github.com/alexreisner/geocoder/issues/816,
  this commit replaces the `sidekiq_worker` example with an example
  using ActiveJob.
* Demonstrate approach to reverse-geocoding records in a background job
  with simple retry logic for timeout and socket errors.

Enable testing different database environments