Fix Travis CI
Fix Travis CI

Through a combination of things, I've gotten Travis to work again.
Not sure which of them fixed it though...
 - Removed mariadb from services (don't need it if it's in addons)
 - Upgraded to 10.3 <-- I think that's the fix
 - Dropped the libmariadbclient-dev installation, because I don't think
   it exists: https://packages.ubuntu.com/search?keywords=libmariadbclient-dev

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