Replace metaclass with singleton_class and some fixes

This change makes cacheify work in the recent environment.

* [Replace `Object#metaclass` with `Object#singleton_class`](https://github.com/vzvu3k6k/cacheify/commit/9cfc479ba26d6408b08f95aa56b00d7e94eabb49) ([`Object#metaclass` is deprecated.](http://apidock.com/rails/Object/metaclass))
* Add some changes in order that `rspec` works without errors or warns.
* [Add Gemfile for bundler](https://github.com/vzvu3k6k/cacheify/commit/2ea1fe1bdade0b844ef0f345a349d5fdd4d3e062) (It's not necessary but many developers will find it useful.)