Switch to use the plaintext formatting for Apache License

It's very clear that the site had copied over the HTML version of the
Apache License.

See [lines 122 to
142](https://github.com/github/choosealicense.com/blob/4e2fd06748ba8cc61c13b604b5bd947242bcbd63/licenses/apache.txt#L122)
as an example where the unordered list bullets not copied in along with
the rest of the text, throwing off the formatting and bunching together
text.

Since the site is displaying the plaintext version of the license, it
makes more sense to use the nicely formatted [plaintext version provided
by Apache](http://www.apache.org/licenses/LICENSE-2.0.txt).