Update to the latest version of RSpec and Bundler.

[treacher]

Context

Due to having an M1 mac I was having issues installing older versions (versions below 2.7) of Ruby and get the tests succeeding locally. To get around this I've updated to the latest version of RSpec and Bundler.

Changes

• Update the Travis CI configuration to test for Ruby 2.7, 3.0, 3.1 and 3.2
• Add rexml gem as it's no longer bundled in newer versions of Ruby.
• Update RSpec to version 3.
• Update bundler version to 2.4.19.
• Update tests to use expect macters.

Testing

Ran tests on Mac (Ruby 2.7) and Ubuntu 20.04 Codespace (Ruby 3.2) and observed they all pass with no deprecation messages.