.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>