Upgrade to Liquibase 3.3.0

This commit updates Spring Boot to Liquibase 3.3.0 and makes the
necessary changes to Boot's PackageScanClassResolver.

Previously, when a class found by the scan could not be loaded a
warning message was logged. This commit lowers these to debug level,
bringing them into line with logging that the standard Liquibase
implementation does. It also avoids a warning always being logged at
startup due to Liquibase's WatchCommand$DynamicContentHandler which
depends on Jetty (an optional dependency of Liquibase).

Closes gh-1382