[ travis ] create experimental directory if non existent

The failed merge of the .travis.yml files had the consequence of
nuking the experimental/ directory. Hence this bug.

Author: gallais

.travis.yml

@@ -134,6 +134,7 @@ script:
       mv html/* . ;
     fi
   - if [[ $TRAVIS_BRANCH = "experimental" ]]; then
+      mkdir -p experimental/ ;
       mv html/* experimental/ ;
     fi