Unix for Poets: Using PDF from Stanford

The previous link pointed to Amazon S3 web space, as mentioned in:
https://github.com/vhf/free-programming-books/pull/909
that does not seem appropriate.
Merge pull request #915 from sebgod/patch-1

Unix for Poets: Using PDF from Stanford
add PHP: The Right Way
Merge pull request #916 from AveVlad/master

add PHP: The Right Way
Remove High Performance Drupal - Not Free
[en] Added CIS 194: Introduction to Haskell
Merge pull request #917 from Pythagory/master

Remove High Performance Drupal - Not Free
Merge pull request #919 from esparta/master

[es] Adding a misc. book
Merge branch 'master' of github.com:guillaume-nargeot/free-programming-books into guillaume-nargeot-master
Correct #918
[en] Sorting Machine Learning section
[en] sorting Ada section
[en] sorting Assambly Language section