Do not install packages which are already installed

Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -22,7 +22,7 @@ before_install:
   - source $HOME/buildtime-trend/init.sh
   - timestamp.sh packages
   - sudo apt-get update -qq
-  - sudo apt-get install git libffi-dev libpq-dev libmysqlclient-dev libicu-dev mercurial
+  - sudo apt-get install git libicu-dev mercurial
 # commands to install dependencies
 install:
   - timestamp.sh install