Treat internal git/http hosts as secure

[sds]

Many organizations have references to internal git repositories in their
Gemfiles. Allow these repositories to be treated as safe since they
are resolvable only internally, and are controlled by the organization.

Closes #72

[sds]

The build failures are related to installing the i18n gem on EOLed versions of Ruby, and are unrelated to this change.

[retornam]

@sds please rebase your commit to kick off the travis test runs again. The i18n gem issue has been fixed.

[sds]

Thanks for the response, @retornam. I've rebased onto the latest master at time of writing (02de26e).

[sds]

Incorporated feedback from @postmodern on https://github.com/sds/bundler-audit/commit/15957412f0c7b1d00b2dd7baac00b82c1d89d680 in d414a12, namely:

• Split out internal_host? helper from internal_ip? to separate URI host lookup from IP subnet-checking logic.
• Check all addresses returned Resolv.getaddresses instead of just the first address returned by Resolv.getaddress

I also added support for IPv6 private addresses.

[postmodern]

An aside, I really wish IPAddress provided a method for determining if an address is from one of the RFC1918 ranges. I recently had to build a list of non-routable address ranges recently.

[bhh]

as it seems people are discussing that topic already in the ipaddress project. ipaddress-gem/ipaddress#54