We recently removed mysqlclient as a dependency in

dev-requirements.txt, in order to simplify getting a development tree

going. We only needed that dependency for some unit tests.
This change introduces a ci-requirements.txt file, which will be used

for any dependencies needed in a CI setup. It currently lists only the

mysqlclient dependency we removed. The Travis CI configuration has

been updated to use it.

Tested that pip installing from the file worked.