Explicitly require IPAddr

[marcotc]

In 0.74.0, #718 was shipped, which introduced a dependency on IPAddr.

This dependency has been transitively loaded by openssl since 2.2.0, released May 13, 2020, which excon always loads. This means that Ruby installations using a version of openssl from before May 13, 2020 can be missing loading ipaddr.

This PR adds that explicitly dependency to the relevant file: lib/excon/connection.rb.

One thing I'm still not sure is why travis-ci is not failing for Ruby 2.3, given I can reliably reproduce it as follows:

Steps to reproduce

$ docker run -it ruby:2.3.8 bash

Then inside the container:

$ git clone https://github.com/excon/excon.git
$ cd excon/
$ bundle install
$ bundle exec shindont

I'm seeing the following error:

  Excon proxy support +++++++++
            tests/proxy_tests.rb
            uninitialized constant Excon::Connection::IPAddr (NameError)
              /excon/lib/excon/connection.rb:516:in `rescue in block in proxy_from_env'

[geemus]

LGTM

[geemus]

Thanks for the heads up and fix.

[geemus]

Released in 0.75.0