Remove old unused functions

Only a select few functions were actually useful, so I've removed all of
the old ones and refactored the good ones:
 - schwartzian_sort -> schwartzian
 - sort_transform -> comparison_fn
 - sort -> comparison
Update array usage to be compatible with PHP 5.3
Fix readme docs
Remove "Graze\" from readme
Remove hhvm-nightly build

Travis-CI couldn't even install hhvm. See
https://travis-ci.org/graze/sort/jobs/36050479