Updated library requires ruby 2.5 which is not provided by default in Travis-ci

[Relequestual]

The linked CVEs show the issue is related to processing XML resulting in a potential DoS.
I don't think this concerns us.

Additionally, the newer version of nokogiri requires ruby 2.5 or above, which is not provided by Travis-ci by default.
We should look into how to request a higher version of ruby.

@dependabot ignore this minor version

Originally posted by @Relequestual in #387 (comment)

[notEthan]

not sure why you say travis does not provide ruby >= 2.5 - it does. ruby 2.7 is current and 2.5 is EOL.

[Relequestual]

I missunderstand this line then: https://travis-ci.org/github/json-schema-org/json-schema-org.github.io/builds/767303313#L409

Can we specify we want a differnet version of ruby than those listed?

[Relequestual]

Given Travis-ci.org is now closed, we should close this issue and create a new one looking to move the CI tests to GitHub actions.