Update CI distro (#6)

* Update CI distro

* no longer needed on Bionic

.travis.yml

@@ -1,4 +1,4 @@
-dist: trusty
+dist: bionic
 sudo: required
 
 env:
@@ -10,7 +10,6 @@ env:
 
 before_install:
   - sudo apt-get -y remove docker docker-engine docker-ce
-  - sudo rm /etc/apt/sources.list.d/docker.list
   - curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
   - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
   - sudo apt-get update