.travis.yml: apt-get update before installing curl

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Bump the syslog-ng requirement to 3.6+

With 3.5/master branched off, bump the syslog-ng requirement to 3.6+.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
.travis.yml: Do not compile master

Master will require syslog-ng 3.6+, which is not packaged yet, so don't
try to compile it for now.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
modules: Convert to syslog-ng master

syslog-ng master changed how LogPipe is created and initialized, adapt
our modules to the new API.

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