Restore testing against lowest allowed versions of dependencies

[deviantintegral]

https://github.com/Lullabot/drainpipe/pull/438/files#diff-be05ff14f4265b6230c383cccc43081eb4dde30ac15ce0e6021de115982724e6L14-L17 removed --prefer-lowest from the test job working with the oldest version of PHP. This allowed us to ensure that transitive dependencies that may be locked to older versions on real-world sites still function, and that we don't accidentally use APIs from newer releases without declaring or updating the dependency.

Let's restore this to our tests.