Fix compatibility of build with latest Travis upgrade

.travis.yml

@@ -35,6 +35,7 @@ notifications:
   on_start: always
 
 addons:
+  firefox: "46.0.1"
   apt:
     packages:
       # upgrade java 8 as the default version 1.8.0_31 prevents from compile sources

travis.sh

@@ -3,7 +3,7 @@
 set -euo pipefail
 
 function configureTravis {
-  mkdir ~/.local
+  mkdir -p ~/.local
   curl -sSL https://github.com/SonarSource/travis-utils/tarball/v31 | tar zx --strip-components 1 -C ~/.local
   source ~/.local/bin/install
 }