Gitignore built directory

[WaveHack]

The source code for this application is written in TypeScript, residing in /issuebot/src/.

The npm start calls the TypeScript compiler, and puts the resulting JavaScript files in built every time you run the application.

I believe it's redundant to have the built directory committed in version control, seeing it gets auto-generated each time you run the app. So I suggest it's a useful candidate to be added to .gitignore.