updates gem dependencies and fixes mimetime fixture failing test

[ryanfelton]

Fixes #3

[summera]

@ryanfelton thanks for the PR! I've fixed the bundler dependency issue in #5 by removing it from the gemspec. I've also committed the Gemfile.lock to source control since that's best practice.

Would you mind rebasing this branch with a focus on fixing the failing tests? I do see that there are some warnings being output when running the tests. I can fix those in a separate PR or if you're interested in tackling that as well, let me know! I don't use this gem anymore, so if you're actively using it, you're probably in a better position than I am to ensure it's working correctly.