fix minitest require in Bundler standalone mode

https://travis-ci.org/lostisland/faraday/jobs/8411907

In Bundler standalone, the `gem` method is still available but the
minitest gem won't be found. Only use the `gem` method when we're
running in the context of Bundler normal mode.
remove void configure_ssl for EventMachine

It never seems to have served any purpose
avoid double loading of certain source files on Ruby 1.8.7

The "already initialized constant NET_HTTP_EXCEPTIONS" and related
warnings would pop up due a circular references triggered by
autoloading.

References #343