Switch to using CircleCI 2.0

We end up using Docker images for PHP 5.6 and MySQL, handle our own
Composer and NPM caches.

.circleci/fake-sendmail.php

@@ -0,0 +1,17 @@
+#!/usr/local/bin/php
+<?php
+$path = "/tmp/fake-mailer";
+if(!file_exists($path)) {
+  mkdir($path);
+}
+$filename = $path . '/mailpoet-' . microtime(true) . '.txt';
+$file_handle = fopen($filename, "w");
+
+$call_arguments = print_r($argv, true) . "\n";
+fwrite($file_handle, $call_arguments);
+
+while($line = fgets(STDIN)) {
+  fwrite($file_handle, $line);
+}
+
+fclose($file_handle);