Fix bug in custom metadata handling

…363)

Fixes #333

When calling to_s will raise, rescue and insert a placeholder.

If a ActionDispatch::RemoteIp instance cannot be memoized, it throws an
IpSpoofAttackError from to_s. In this case, replace the IP with a
placeholder.

Ref:
https://github.com/rails/rails/blob/34fe2a4fc778d18b7fe6bdf3629c1481bee789b9/actionpack/lib/action_dispatch/middleware/remote_ip.rb#L134

Fixes #351

…)"

This reverts commit c7862ea.

Fixes #357
Closes #358

NameError: uninitialized constant Report::UNHANDLED_EXCEPTION_MIDDLEWARE

In newer versions of Ruby using File.exists? will warn about deprecation. The
File.exist? method has been around forever so this will not introduce any
backwards compatibility problems.

Fixes #371 allow transparent proxy config in `Net::HTTP` via ENV

* Fixes some travis shenanigans with upgrading to latest rubygems
* Removes stray files

Merge pull request #380 from dexhorthy/net-http-proxy