travis: change how docker gpg key is downloaded (spotify#865)

Requests to p80.pool.sks-keyservers.net are timing out in recent travis
builds. Use the ubuntu key server.

.travis.sh

@@ -23,7 +23,7 @@ case "$1" in
     sudo rm -fr /var/lib/docker || :
 
     # As instructed on http://docs.master.dockerproject.org/engine/installation/linux/ubuntulinux/
-    sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
+    sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
     sudo sh -c "echo deb https://apt.dockerproject.org/repo ubuntu-trusty main > /etc/apt/sources.list.d/docker.list"
     sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"