fix: [travis] Try and require xenial (16.04) and see if it works better.

.travis.yml

@@ -12,9 +12,7 @@ env:
       AIL_FLASK=$TRAVIS_BUILD_DIR/var/www/ AIL_REDIS=$TRAVIS_BUILD_DIR/redis/src/ \
       AIL_LEVELDB=$TRAVIS_BUILD_DIR/redis-leveldb/ PATH=$AIL_HOME:$AIL_REDIS:$AIL_LEVELDB:$PATH
 
-
-before_install:
-    - sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6B05F25D762E3157
+dist: xenial
 
 install:
     - ./installing_deps.sh