.travis.yml: Add a missing dependency

[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>

[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

[nastasi]

LGTM