Switching Travis CI builds to Ubuntu Bionic Beaver

According to the [documentation](https://docs.travis-ci.com/user/reference/trusty/#container-based-infrastructure), `sudo: false` is no longed needed since Trusty.

Test against PHP 7.4 on AppVeyor

Switching Travis CI builds to Ubuntu Xenial Xerus

Test primarily against PHP 7.4 on Travis

Fix unquoted stmt fragments backslash escaping

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

Use PHP 7.4 instead of a snapshot on Travis

Develop branch is gone

Dropped SQL Server 2008 support

Chocolatey returns only 7.4.0 in the search results but there's no SQL Server extensions released for it yet.

Locked the PHP_CodeSniffer version to 3.5 for better compatibility with PHP 7.4

PingableConnection and ServerInfoAwareConnection now extend Connection

VersionAwarePlatformDriver now extends Driver

Manually merge 2.10 into master

Switched from PHPBrew-based configuration to the Docker-based

As per the support response, continuousphp switched to building their containers based on the official images from PHP.

Remove temporary SQLite file on teardown

Update deprecation messages to refer to DBAL

Fix of branch names for doctrine website config

Fix upgrade comment about MariaDB

Generally advising to upgrade MariaDB to PostgreSQL isn't helpful ;)

Add .doctrine-project.json to .gitattributes