Update AppImage workflow file

Set ubuntu dependencies to the packages that exist on Ubuntu Focal

.travis.yml

@@ -1,10 +1,10 @@
 language: cpp
 compiler: gcc
 sudo: require
-dist: trusty
+dist: focal
 
 install:
-  - sudo apt-get -y install pkg-config libglib2.0-dev libgconf2-dev libgtk2.0-dev libwnck-dev libwxgtk3.0-dev libxcb1-dev libxcb-ewmh-dev xcb-proto librsvg2-dev
+  - sudo apt-get -y install pkg-config libglib2.0-dev libgconf2-dev libgtk-3-dev libwnck-3-dev libwxgtk3.0-gtk3-dev libxcb1-dev libxcb-ewmh-dev xcb-proto librsvg2-dev
 
 script:
   - make -j$(nproc)