.travis.yml: Use aptitude to install syslog-ng-dev

Since syslog-ng-dev depends on syslog-ng-core, which conflicts with
rsyslog, upon with ubuntu-minimal depends, use aptitude to install
syslog-ng-dev, because it can make the right decision.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Makefile.am: Add DISTCLEANFILES

To make distcheck happy, clean up the generated grammar files. We use
some GNU Make trickery to do that, so we won't have repeated code all
over the place.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>