Use `textContent` when `innerText` is not available.

Firefox does not implement `innerText` so this led to another `TypeError`.
Merge pull request #518 from twbs/innerText

Use `textContent` when `innerText` is not available.
Update "grunt-contib-concat" dependency.
Grunt.
Use plain `if/else` statements when there's no assignment.
Switch to `(function){..})()`.
Remove JSHint's `expr` rule.
Break function.
Run `grunt`.