Convert to Ruby2D

[dealingwith]

closes #7

Originally I was just trying to manage the project's dependencies by putting the previous library, rubygame in a Gemfile so we can use bundler to manage dependencies. Then I discovered rubygame is ~10 years old and unmaintained. This caused 🚨 so I went back to Ruby2D to see if we could use it instead.

It installed easily and ran the first time. ✨ I was able to quickly port @jamespeilunli's code from #11 to Ruby2d 🎉

So this PR:

• adds Gemfile
• refactors tetris.rb and tetrominos.rb to ruby2d

TODO:

• [x] Figure out why two tetrominoes are being drawn in yellow (I think the orange one is yellow).
• [x] Write instructions in README.md

[dealingwith]

@jamespeilunli note I've converted indentation to 2 space

[jamespeilunli]

Looks like that reinstall of Ubuntu helped something; Ruby2D works for me now.

[dealingwith]

Did you follow the instructions in the README and use Bundler?

[jamespeilunli]

Sorry, I didn't see the README.

I saw that Ruby2d used SDL (in Linux), and I thought I had all the dependencies I needed since Rubygame also used SDL. But, when I ran sudo gem install ruby2d, it alerted me that I needed to install these packages: libsdl2-dev libsdl2-image-dev libsdl2-mixer-dev libsdl2-ttf-dev, so I ran sudo apt-get install *those packages*. Running sudo gem install ruby2d after that threw no errors, and I tested it out on a test file with the example program on the Ruby2d homepage and it worked.