Bump to version 2.0.18

Fix backward-compatibility with vlc 2.x and solve some issues with
mouse wheel and DVD menu navigation:

• Start using TravisCI for test builds
• mediawidget: Override DVD keys instead of changing their settings
• mediawidget: warn user to use keys for DVD menu
• mediawidget: remove a now uneeded check
• mediawidget: fix video position movement with mouse wheel
• mediawidget: fix the mouse wheel behavior at the streaming window
• vlc: with vlc 2.x, use vlcMedia for libvlc_MediaMetaChanged event
• vlc: fix support for vlc 2.x

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>