QUA-474: update phpmd (#58)

Style changes docs (#57)

Rename circleci release job to release_images (#56)

Revert "Attempt to make CircleCI builds run again."

This reverts commit 0ad60b6.

Revert "Refactor repeated strings into constants, as per Sonar."

This reverts commit baef18b.

Update README.md (#40)

Headers weren't displaying due to malformed markdown.

Fixes exception bubbling for php7 (#39)

Rescues `Throwable` which includes `Exception` and php7 `Error` objects.

Supports string and array settings for file_extensions config (#37)

* Fixes Dockerfile and Makefile to update components

* Supports string and array settings for file_extensions config

Avoid printing a php notice for each issue

The interpreter wants us to do this `end` shenanigan. Getting this
method under test actually _required_ us to avoid the notice, which is
kind of cool.

Dockerfile fixes (#35)

* add php7 package

Otherwise there's no /usr/bin/php7 and nothing works

* Improve development workflow in Dockerfile

We should leverage docker caching so builds take less time.