Heroku supports PHP. Fix link.

Heroku PHP support is now GA.
https://blog.heroku.com/archives/2014/7/15/the_new_php_on_heroku_now_generally_available
Merge pull request #420 from iakio/patch-1

Heroku supports PHP. Fix link.
Merge remote-tracking branch 'upstream/gh-pages' into gh-pages