Fix test failures from using latest version of dependencies

[dylanahsmith]

Closes #182, closes #183, closes #184 by updating the Gemfile.lock

Problem

The latest CI build on master is failing with File does not exist: mocha/mini_test for ruby 2.3 to 2.5. Ruby 2.1 and 2.2 were failing to install bundler.

Solution

• I changed the 'mocha/mini_test' to 'mocha/minitest' to reflect the renamed file upstream
• I added a require 'socket' to fix TCPServer constant not found errors
• I removed end-of-life versions of ruby from CI (the next release will be a breaking change anyways) and added ruby 3.0
• I removed a server error expectation that no longer gets an error, which seems like it is due to memcached server changes. It doesn't seem like it is testing a different code path in the client anyways.

[arthurnn]

❤️

[arthurnn]

thanks Dylan, i added you as a collaborator, go ahead and merge the other PRs ❤️