.remark-slide { table-layout: fixed; }

This makes `img { max-width: 100%; }` work in firefox (and not just
chromium).

I had to skip the tests when building since `make test` resulted in

```
  1) SlideView background should be set from background slide property:
      
      actual expected
      
      image.jpg
      
      at Object.Assertion.equal (/home/felix/dev/forks/remark/node_modules/should/lib/should.js:303:10)
      at Context.<anonymous> (/home/felix/dev/forks/remark/test/remark/views/slideView_test.js:26:63)
      at Test.Runnable.run (/home/felix/dev/forks/remark/node_modules/mocha/lib/runnable.js:213:32)
      at Runner.runTest (/home/felix/dev/forks/remark/node_modules/mocha/lib/runner.js:351:10)
      at /home/felix/dev/forks/remark/node_modules/mocha/lib/runner.js:397:12
      at next (/home/felix/dev/forks/remark/node_modules/mocha/lib/runner.js:277:14)
      at /home/felix/dev/forks/remark/node_modules/mocha/lib/runner.js:286:7
      at next (/home/felix/dev/forks/remark/node_modules/mocha/lib/runner.js:234:23)
      at Object._onImmediate (/home/felix/dev/forks/remark/node_modules/mocha/lib/runner.js:254:5)
      at processImmediate [as _immediateCallback] (timers.js:330:15)
```

before I even modified anything.

\- Felix