Remove use of rest-client since it's no longer maintained

[Oneill38]

It looks like the rest-client gem no longer has any active maintainers and has not had a commit since 2019. A different http lib should be used that has active maintainers who are fixing bugs and security issues.

[jmlagace]

The bigger problem I see (until they have a replacement is that version 2.17 now has a restriction on version 2.0.2 of rest-client when older versions (like 2.8.6) worked perfectly fine with the version of 2.1.0 of rest-client.

I don't see how going back to an older version makes any sense.

[lilith]

Given how frequently http vulnerabilities pop up, this is really neglectful of Chargebee.