Use error patterns in php-phpcs

Parsing XML is nicer in theory, but in practice PHP has notoriously
unstable output habits which tend to break XML parsing.

See #78 and #118.
[Fix #115] Remove Pyflakes

Flake8 is superior in all aspects
Improve speed of Python provisioning

Use a simpler index to avoid expensive link crawling on the standard index.
Improve wording in changelog

[ci skip]
Build docs on Travis

Ensures that our docs build cleanly
Update Carton version
Add argument symbol for temporary directories

Syntax checkers can use this symbol to pass a temporary directory to the
executable.