CI: Fix building on trusty

CI/install-dependencies-linux.sh

@@ -2,6 +2,7 @@
 set -ex
 
 sudo add-apt-repository ppa:jonathonf/ffmpeg-3 -y
+curl -L https://packagecloud.io/github/git-lfs/gpgkey | sudo apt-key add -
 sudo apt-get -qq update
 sudo apt-get install -y \
         build-essential \