Replace dashes in Bundle's config with triple underscore.

From the following warning when running bundle nowadays:

Your /home/nimrod/.bundle/config config includes
`BUNDLE_HTTPS://RUBYGEMS__PKG__GITHUB__COM/SMILE-IO/`, which contains
the dash character (`-`).
This is deprecated, because configuration through `ENV` should be
possible, but `ENV` keys cannot include dashes.
Please edit /home/nimrod/.bundle/config and replace any dashes in
configuration keys with a triple underscore (`___`).