Use W3C compliant `textContent` instead of `innerText

Closes https://github.com/github/choosealicense.com/issues/119, closes https://github.com/github/choosealicense.com/issues/46.

Firefox doesn't have `innerText`, but it does have `textContent`. This change allows the button to say "Copied!" when click on, and, on mouseleave, it goes back to the default text. This was also tested and working in Chrome.

![ff](https://cloud.githubusercontent.com/assets/64050/5494793/aaaf8926-86ac-11e4-993b-ccab4aa1458c.gif)

It's a minimal change, but still: /cc @github/choosealicense-com