Update Firefox ES6 data.

Firefox 27 and 28 share the same feature set with regard to ES6 support.
The current Fx28 column has been merged with Fx27 and subsequently
renamed to Fx29.

Meanwhile, Firefox 17 ESR was retired in favor of Firefox 24 ESR. Thus,
Fx27 has been marked as obsolete.
Merge pull request #101 from fbender/gh-pages

Update Firefox ES6 data