chore(dependencies): revert to marked@0.2.9 to fix CI builds

marked v0.2.10 breaking test:docgen

Tests on travis have been failing the past few days, so I wanted to dig around for the cause.

To reproduce:
```bash
rm -rf node_modules/marked
npm install marked@0.2.9
grunt test:docgen

(PASSES)

rm -rf node_modules/marked
npm install marked (now installs 0.2.10)
grunt test:docgen

(FAILS)
```

I think the simplest way to fix travis build would be to remove the `~` operator before the version number in package.json, but a more robust fix would involve migrating the code. Clearly there are some breaking changes introduced in v0.2.10