changelog: use time elements for release dates

The markup change is purely semantic; the site's appearance is unchanged. Using time elements (with some straightforward styling) seems cleaner than the current small+i combo.

To ensure I didn't introduce any typos, I wrote a couple of scripts and compared their output.

```bash
git tag | xargs -I {} \
  git --no-pager log --max-count=1 --pretty=format:'%d | %ad | %ad
' --date=short --decorate=short {} | \
  sed -e 's/ (tag: \(.*\))/\1/' \
      -e 's/ \([0-9]*\)-0*\([0-9]*\)-0*\([0-9]*\)$/ \2 \3, \1/' \
      -e 's/ 1 / January /' \
      -e 's/ 2 / February /' \
      -e 's/ 3 / March /' \
      -e 's/ 4 / April /' \
      -e 's/ 5 / May /' \
      -e 's/ 6 / June /' \
      -e 's/ 7 / July /' \
      -e 's/ 8 / August /' \
      -e 's/ 9 / September /' \
      -e 's/ 10 / October /' \
      -e 's/ 11 / November /' \
      -e 's/ 12 / December /'
```

```javascript
_.chain(document.querySelectorAll('[id]'))
  .filter(function(el) {
    return /^\d\.\d\.\d$/.test(el.id);
  })
  .map(function(el) {
    var time = el.querySelector('time');
    return [el.id, time.getAttribute('datetime'), time.innerHTML].join(' | ');
  })
  .value()
  .reverse()
  .join('\n');
```

I noticed that the 1.4.2 release date was incorrect, so fixed it in an isolated commit. The only remaining differences are 0.5.3 and 1.3.2 which, though tagged, do not appear in the change log. I believe this is intentional.