[Travis] Rework assimp dependency

.travis.yml

 language: cpp
-sudo: required
 
 os:
   - linux

ci/install_linux.sh

@@ -7,9 +7,7 @@ sudo apt-get -qq update
 sudo apt-get -qq --yes --force-yes install cmake
 sudo apt-get -qq --yes --force-yes install libboost-all-dev
 sudo apt-get -qq --yes --force-yes install libccd-dev
-
-# Assimp
-sudo apt-get install libassimp-dev
+sudo apt-get -qq --yes --force-yes install libassimp-dev
 
 ########################
 # Optional dependencies