.travis.yml: Add a missing dependency

Created by: dshcherb

libwebkit2gtk-3.0-dev is required to build Remmina on trusty via

cmake -DCMAKE_BUILD_TYPE=Debug -DWITH_SSE2=ON --build=build <src_dir>

Signed-off-by: Dmitrii Shcherbakov <fw.dmitrii at gmail.com>

Created by: dshcherb

Couldn't get it working without the missing dependency on a clean trusty container:

cmake -DCMAKE_BUILD_TYPE=Debug -DWITH_SSE2=ON -DWITH_APPINDICATOR=OFF --build=build .. -- The C compiler identification is GNU 4.8.4 -- Check for working C compiler: /usr/bin/cc -- Check for working C compiler: /usr/bin/cc -- works ... -- checking for modules 'avahi-ui-gtk3>=0.6.30;avahi-client>=0.6.30' -- found avahi-ui-gtk3, version 0.6.31 -- found avahi-client, version 0.6.31 -- Found AVAHI: TRUE
-- Enabling Survey. -- checking for one of the modules 'webkit2gtk-4.0;webkit2gtk-3.0' CMake Error at remmina/CMakeLists.txt:124 (message): Either webkit2gtk-4.0 or webkit2gtk-3.0 is required

Created by: nastasi

LGTM